US 20030101891 A1
Jacketed firearms projectile having a jacket thickness less than approximately 0.025 inches thick are described, where the density of the projectile core has been selected so that the jacketed projectile has a weight that is at least substantially the same as a reference firearms projectile, such as a lead projectile, having substantially the same size and shape. In one aspect, the projectile is a frangible projectile. In another aspect, the projectile is lead-free. In yet another aspect, the projectile is incorporated in a cartridge. Methods for forming the projectile and forming cartridges containing the projectiles are also disclosed.
1. A frangible bullet having a size, shape, and weight, the bullet comprising:
a frangible bullet core having a bullet core density; and
a frangible bullet jacket comprising a jacketing material that at least substantially encloses the bullet core, wherein the jacket has a jacket thickness in the range of approximately 0.005 inches and approximately 0.025 inches; and
wherein the bullet core has a core density that is selected so that the size, shape, and weight of the frangible bullet is at least substantially the same as a corresponding size, shape, and weight of a reference bullet having a core that is at least substantially formed from lead.
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25. A lead-free bullet having a size, shape, and weight, the bullet comprising:
a lead-free bullet core having a bullet core density;
a bullet jacket comprising a jacketing material that at least substantially encloses the bullet core, wherein the jacket has a jacket thickness in the range of approximately 0.005 inches and approximately 0.025 inches; and
wherein the bullet core has a core density that is selected so that the size, shape, and weight of the lead-free bullet is at least substantially the same as a corresponding size, shape, and weight of a reference bullet having a core that is at least substantially formed from lead.
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40. A method of forming a jacketed bullet, comprising the steps of:
selecting a reference bullet having a desired bullet size, bullet shape, and bullet weight, where the reference bullet includes a bullet core having a reference bullet core composition;
selecting a jacket material having a jacket density for the jacketed bullet;
selecting a jacket thickness for the jacketed bullet;
calculating a core density, a core size, and a core shape for a target bullet core, so that when the target bullet core is jacketed by the selected jacket material at the selected jacket thickness the resulting jacketed bullet will have the desired bullet size, desired bullet shape, and desired bullet weight;
preparing the target bullet core having the calculated core density, size, and shape, where the target bullet core composition is distinct from the reference bullet core composition; and
jacketing the target bullet core with the selected jacket material at the selected jacket thickness to produce the jacketed bullet.
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 A firearms projectile, or bullet, constructed according to the present invention is shown in FIG. 1 and indicated generally at 10. Bullet 10 includes a bullet core 12 and a protective coating, or jacket, 14. It should be understood that the illustration of bullet 10 in FIG. 1 is exemplary and that the shape and dimensions of bullet 10 may vary, such as to conform to the shape and size of conventional firearms projectiles. It should be understood that the depicted thickness of the jacket and relative thickness of the jacket compared to the overall size and shape of the bullet in FIG. 1 are exaggerated for the purposes of illustration.
 In FIG. 2, bullet 10 is shown forming part of a firearms cartridge 20. As shown, cartridge 20 includes bullet 10, a case or casing 16, that includes a casing neck 15, a propellant charge 18, and a priming mixture or primer 19. Casing 16, primer 19 and propellant charge 18 may be of any suitable materials, as is known in the art of firearms. Cartridge 20 is ready to be loaded into a gun, such as a handgun, rifle or the like, and upon firing, discharges projectile 10 at high speeds and with a high rate of rotation. Cartridge 20 is depicted as a centerfire cartridge, in which primer 19 is located in the center of the base of casing 16. However, bullet 10 is optionally incorporated into a firearms cartridge that is a rimfire cartridge, where the casing is rimmed or flanged and the primer is located inside the rim of the casing.
 Bullets are commonly expelled from firearms with rotational speeds greater than 10,000 rpm. This rapid rate of rotation imparts significant centrifugal forces on the bullet itself. Protective jacket 14 prevents these centrifugal forces from fragmenting, obturing (deforming on account of centrifugal forces), and/or dispersing bullet core 12 during flight. The jacket may partially or completely enclose the bullet core. Preferably, jacket 14 at least substantially encloses bullet core 12. Examples of suitable materials for jacket 14 include copper and alloys containing copper, although other materials may be used.
 Jacket 14 on bullet 10 is typically thinner than the jacket found on a conventional jacketed bullet having substantially the same size and shape. The use of a thinner jacket results in thinner and smaller jacket fragments upon impact than those resulting from the impact of a conventional jacketed bullet having substantially the same size and shape, thereby decreasing the risk of injury from jacket fragments ricocheting or rebounding from the target surface. In one embodiment of the invention, jacket 14 has an average thickness that is less than approximately 0.025 inches. In some embodiments, jacket 14 has an average thickness that is less than approximately 0.01 inches. In another embodiment, jacket 14 has an average thickness in the range of approximately 0.005 inches and approximately 0.01 inches. In a preferred embodiment, the jacket thickness is in the range of approximately 0.006-0.008 inches.
 Although jackets having a thickness less than approximately 0.005 have been prepared, many firearms have rifling lands up to about 0.004 inches in height. As the bullet travels through the barrel of a firearm, the rifling present in the barrel imparts a high rate of rotation to the bullet. As discussed above, any weakening of the jacket by deep grooves cut into the jacket by the rifling may permit centrifugal forces to tear the jacket from the bullet core. Additionally, any grooves cut by the lands through the projectile jacket may expose the bullet core, resulting in possible premature fragmentation of the bullet, and may also result in increased barrel fouling. Bullets having a jacket thickness less than 0.005 are therefore not preferred, although they could be compatible with a firearm having more shallow rifling. Expressed another way, jacket 14 may have a range of thicknesses, and preferably has a thickness that exceeds the height of the rifling lands by at least approximately 0.001 inches, and in many embodiments, by at least approximately 0.002 to approximately 0.004 inches.
 In order to match the ballistic characteristics of another projectile, the projectile of the invention has substantially the same overall size, shape and weight as a reference projectile. As used herein, a “reference” projectile or bullet is a projectile or bullet having a desired size, shape and weight. The size, shape, and weight of a reference projectile is typically selected to facilitate the use of the projectile in a conventional firearm, as well as conferring favorable ballistic characteristics on the projectile.
 Matching the ballistic characteristics of a reference bullet includes substantially matching, for example, one or more of the velocity, momentum, kinetic energy, bullet drop, bullet trajectory, target lead, and bullet time of flight, among other characteristics. Preferably several, if not all or substantially all of these characteristics are at least substantially matched. Examples of lead-free and/or frangible bullets have been prepared using materials having a lower density than conventional bullets, thereby resulting in a bullet having less weight. Such bullets often fail to match the point of impact of conventional bullets, and often produce a diminished recoil upon firing, both factors effectively compromising the value of target practice using such bullets as preparation for the use of conventional ammunition. One method of attempting to compensate for the decreased weight of the bullet has been to increase the propellant charge used, thereby providing an enhanced recoil and improved point of impact characteristics. However, the use of increased propellant charges may alter bullet velocities, bullet times of flight, bullet drop, or other ballistic characteristics. Similarly, increasing the propellant change also tends to increase barrel wear and/or damage. Another method of attempting to compensate for the decreased weight of the bullet is to make the bullet longer than a corresponding reference bullet. However, lengthening the bullet increases the friction and surface area of contact with the rifle or other firearm and thereby results in a corresponding increase in barrel pressure and decrease in muzzle velocities. To maximize the effectiveness of target practice, the bullet used should have a weight and propellant charge comparable to that of a conventional bullet in order to reproduce the ballistics of that conventional bullet.
 The jacketed projectiles of the present invention are manufactured so that the projectile has a size, shape, and weight that is at least substantially equivalent to that of a reference projectile. By matching reference projectile weight as well as size and shape, an equivalent propellant charge may be used, resulting in substantial consistency in ballistic characteristics between firing a projectile of the invention and the reference projectile.
FIG. 3 shows schematic cross-sections of four bullets. Bullet 10 corresponds to a bullet of the invention, while bullets 30, 40 and 50 correspond to selected reference bullets. The selected reference projectile may be prepared from any suitable material or materials. The overall size and shape of each projectile is the same, however the relative jacket thicknesses have been exaggerated for the purposes of illustration.
 In one aspect of the invention, the reference projectile includes a bullet core containing lead or a lead-alloy, such as a lead-antimony alloy. Bullet 30 of FIG. 3 is a conventional lead-containing jacketed bullet, where the thickness of jacket 34 is greater than that of jacket 14. Provided that the density of the jacket material is less than that of the bullet core, as is the case for copper jackets, it should be apparent that in order for bullet 10 to have an overall weight that is equivalent to that of reference bullet 30, the density of bullet core 12 will have to be somewhat less than that of lead-containing bullet core 32, provided that the same jacket material is used in both bullets. This is due to the greater percentage of the total bullet volume of bullet 10 that is taken up by the bullet core. In order to match the weight of reference bullet 30, therefore, the bullet core 12 should be manufactured to have a correspondingly reduced density.
 Bullet 40 of FIG. 3 is a conventional lead-containing jacketed bullet where the thickness of jacket 44 is less than that of jacket 14. Again, provided that the density of the jacket material is less than that of the bullet core, it should be apparent that in order for bullet 10 to have an overall weight that is equivalent to that of reference bullet 40, the density of bullet core 12 will have to be somewhat greater than that of lead-containing bullet core 42, provided that the same jacket material is used in both bullets.
 Bullet 50 of FIG. 3 is a conventional lead-containing bullet that is not jacketed. In order for bullet 10 to have an overall weight that is equivalent to that of bullet 50, the density of bullet core 12 will have to be greater still, at least greater than that of bullet core 52, provided that the jacket material used to prepare jacket 12 has a density less than that of bullet core 54. The greater the difference in densities between the material of jacket 12 and the material of bullet core 52, the greater the difference in densities between bullet core 12 and bullet core 44 in order to achieve an equivalent overall weight.
 Desired Bullet Core Density Determination
 Before a bullet of the invention can be manufactured, a reference bullet having a desired bullet size, bullet shape, and bullet weight must be selected. The reference bullet may be jacketed or non-jacketed, and the jacket material may be copper, copper alloy, or other metal. Alternatively, the jacket material may be a non-metallic polymer, such as nylon or TEFLON™. The bullet core composition of the reference bullet may include lead or lead alloys, such as lead-antimony alloy. Alternatively, the bullet core composition of the reference bullet may be substantially lead-free, having a density either greater or lesser than lead or lead alloy.
 Upon selection of the reference bullet, the size, shape, and overall weight of the desired bullet, or “target” bullet, are determined. The target jacket thickness and jacket material are similarly selected. Once the dimensions of the bullet have been determined, and the jacket thickness and density have been selected, the size, shape and density of the target bullet core can be calculated to obtain a target bullet that matches the size, shape, and weight of the reference bullet.
 It should be apparent to a skilled artisan that a variety of methods exist for modeling and/or calculating the appropriate bullet core density, ranging from simple physical methods of measuring volume and density such as displacement measurements and mass determinations for both the reference bullet and the target jacket, to sophisticated methods that utilize mathematical and/or computer generated models of the target bullet in order to determine the appropriate target bullet core density.
 A particular simplified bullet model permits the target bullet core density to be calculated by making the following assumptions and approximations for the both the target and reference bullets:
 The overall bullet weight and external dimensions are the same for both the conventional jacketed bullet and the bullet of the invention.
 Each bullet has a uniform jacket thickness.
 At any given slice perpendicular to the long axis of the bullet, the weight-per-unit length is equal for both the conventional jacketed bullet and the bullet of the invention.
 Relying upon the above assumptions, and by setting the width of each perpendicular slice to an arbitrary unitary value, the following equation may be used to calculate the desired bullet core density:
 where dcn is the desired bullet core density, dcl is the conventional bullet core density, dj is the density of the jacket material of both bullets, tl is the conventional bullet jacket thickness, tn is the desired jacket thickness of the new bullet, and Dj is the outer diameter of each bullet.
 The density of lead is 11.3 grams/cm3, while lead alloys used for bullet cores may have a density of 11.2 to 11.0 grams/cm3. For the purposes of an exemplary calculation, the density of a lead-antimony alloy bullet core (dcl) is considered to be 11.2 grams/cm3, while the density of traditional copper jacketing is approximately 8.96 grams/cm3 (dj). Using these values, the desired core density for a bullet of the invention may be calculated using the above formula.
 A conventional 0.223 caliber, 55 grain rifle bullet has a traditional jacket thickness (tl) of 0.020 inches, and a major bullet diameter (Dj) of 0.223 inches. If the new jacketed bullet is to have a jacket thickness (tn) of 0.007 inches, and a substantially equal overall weight, then the desired bullet core density is calculated to be 10.68 grams/cm3, significantly less than that of lead or lead alloy.
 Similarly, a conventional 9 mm, 115 grain pistol bullet has a jacket thickness of 0.012 inches and a major bullet diameter of 0.357 inches. If the new jacketed bullet is to have a jacket thickness of 0.007 inches, and a substantially equal overall weight, then the desired bullet core density is calculated to be 11.07 grams/cm3.
 A conventional 0.45 caliber, 230 grain pistol bullet has a jacket thickness of 0.015 inches, and a major bullet diameter of 0.45 inches. If the new jacketed bullet is to have a jacket thickness of 0.007 inches, and a substantially equal overall weight, then the desired bullet core density is calculated to be 11.04 grams/cm3.
 The same approximations and simplified equation can be used to calculate the desired bullet core density when the jacketed bullet of the invention is intended to match the weight and other characteristics of an unjacketed conventional bullet. In this instance the jacket thickness of the conventional bullet (tl) is set to zero. For example, attempting to match the overall weight of an unjacketed 0.357 caliber pistol bullet in a new bullet having a copper jacket with a thickness of 0.007 inches results in a desired bullet core density of 11.29 grams/cm3, greater than that of the lead alloy of the conventional bullet (11.2 grams/cm3).
 The relationship between bullet size, jacket thickness, and desired bullet core density is shown more clearly in FIG. 4, which illustrates the effect of bullet size on desired core density when jacket thicknesses are held constant. The curve of FIG. 4 is generated by setting the desired jacket thickness at 0.007 inches, and calculating the bullet core density required in order to match the ballistic characteristics of a conventional jacketed bullet having a jacket thickness of 0.020 inches. As is shown in the plot, the larger the proportion of overall bullet volume that is taken up by jacketing material in the conventional bullet, the greater the compensation that must be made in bullet core density to achieve an equivalent overall weight when a thinner jacket is utilized, and therefore achieve substantially the same ballistic characteristics as the target bullet.
 Jacketed Bullet Manufacture
 Once the appropriate desired bullet core density has been determined, the bullet core must be manufactured so that it exhibits the desired density, and is at least substantially enclosed by the appropriate jacket.
 As discussed above, the bullet or projectile is optionally frangible. Although the frangible bullet of the invention may simply break into several pieces upon impact, it is preferred that the frangible bullet of the invention is at least substantially reduced to powder upon impact, and even more preferable that the projectile is completely reduced to powder upon impact. By “substantially reduced to powder” it is meant that at least 50% of the bullet core is reduced to powder. It is within the scope of the present invention, however, that bullet 10 may also be an infrangible bullet, which is designed to remain in a single piece after impact.
 As discussed above, the bullet or projectile is optionally lead-free, so that the overall bullet presents less of a toxicity hazard during and after use. In some embodiments, the bullet or projectile is both lead-free and ferromagnetic. In a preferred embodiment, the bullet of the invention is both lead-free and frangible.
 Lead-free bullets and cartridges have been previously described in provisional patent application Serial No. 60/260,626 filed Jan. 9, 2001 by Darryl D. Amick, hereby incorporated by reference. In particular, bullets prepared from tin and tungsten powders have been previously described in provisional patent application Serial No. 60/296,267 filed Jun. 5, 2001 by Darryl D. Amick, also incorporated by reference.
 The bullet core of the invention typically includes a metal or metal alloy. In one aspect of the invention, the bullet core includes a metallic powder and a binder, where the binder may be a metallic binder or a non-metallic binder. Where the bullet core of the invention is formed from a metallic powder and a binder, they are typically mixed together and then compressed to match both the desired shape and density. Typically, molds or dies, such as profile dies, are used to define the shape of the projectile, with compressive rams being used to apply the determined amount of compression. The metallic powder may include one or more metals or metal alloys.
 The bullet core of the invention may optionally contain a relatively small component, such as between approximately 0% and approximately 1%, of a suitable lubricant, such as to facilitate easier removal of the bullet from a die. An example of a suitable lubricant is ACRAWAX™ dry lubricant, although others may be used. When selecting suitable combinations of powders and binders, as well as suitable actuating and/or compression values, such factors to watch include the hardness, brittleness, lubricity, and spalling of the resulting bullet.
 In some embodiments, the powder used to prepare bullet core 12 may be selected to be ferromagnetic, such as by including ferrotungsten, so that the resulting projectile is magnetic. This enables the projectile to be reclaimed using a magnet. For example, magnets may be used to recover magnetic projectiles from bodies of water and from shooting ranges. Such a projectile may also be referred to as a recyclable projectile because it is easily reclaimed. When bullet 10 is both frangible and metallic, the magnetic powder remaining after impact may be easily reclaimed, or recycled, using a magnet. In addition, this reclamation process also enables the powder to be separated from non-magnetic objects, such as loose binder, casings, jacketing materials, portions of targets and other debris, and the like. The recovered metallic powder may then be recycled to produce additional bullets. Using a ferromagnetic metallic powder also enables an easy determination, using a magnet, that the bullet is not formed from lead, which is not magnetic. Although ferromagnetic powders may be desirable in some applications, it is also within the scope of the present invention that metallic powders may be used that are not ferromagnetic.
 Examples of particularly well-suited metallic powders are powders containing tungsten or tungsten alloys. Examples of suitable tungsten alloys are collectively referred to as “WHA's” (tungsten heavy alloys) and have densities in the range of approximately 15 grams/cm3 to approximately 18 grams/cm3. Pure tungsten has a density of 19.3 grams/cm3. These powders are especially well-suited for use in preparing the bullet core of the invention because they can be mixed with less dense materials, such as an appropriate binder, to produce a resulting bullet having a desired density, even where the density is greater than that of lead or lead alloys. Examples of suitable tungsten alloys include, but are not limited to, W—Cu—Ni, W—Co—Cr, W—Ni—Fe, W—Ni, tungsten carbide, W—Fe (ferrotungsten) and alloys of tungsten with one or more of nickel, zinc, copper, iron, manganese, silver, tin, bismuth, chromium, cobalt, molybdenum and alloys formed therefrom, such as brass and bronze. Still further examples of suitable compositions for metallic powder include powders formed from 73.64% WHA and 26.36% iron; 70% WHA and 30% zinc; 80% WHA and 20% zinc; 80% WHA, 19% zinc and 1% lubricant; 68% WHA and 32% copper; 68% WHA, 31.5% copper and 0.5% lubricant; 70% WHA and 30% tin; and 70% WHA, 29.5% tin and 0.5% lubricant. The individual metallic powders may be obtained in various coarseness, or mesh-sizes.
 Another suitable metallic powder is powder produced from recycled tungsten and tungsten alloys, such as waste materials formed when tungsten or tungsten alloys are forged, swaged, drawn, cropped, sawed, sheared, and machined. Operations such as these inherently produce a variety of metallic scrap, such as machine turnings, chips, rod ends, broken pieces, rejected articles, etc., all of which are generated from materials of generally high unit value because of their tungsten content. Suitable processes for obtaining this powder, and compositions of such powder are disclosed in co-pending U.S. patent application Ser. No. 09/483,073, which is entitled “Methods for Producing Medium-Density Articles from High-Density Tungsten Alloys,” was filed on Jan. 14, 2000, and the complete disclosure of which is hereby incorporated by reference. Still other suitable metallic powders are tungsten-containing powders that have been high-energy milled with one or more other metallic powders to produce mechanical alloying effects, as disclosed in U.S. Pat. No. 6,248,150 to Amick (2001), the complete disclosure of which is hereby incorporated by reference.
 The bullet core of the invention optionally includes a binder, that may be a metallic binder or a non-metallic binder. Where the binder is a non-metallic binder, it is optionally a polymeric binder.
 Polymeric Binders.
 In one aspect of the invention, the bullet core is prepared from a metallic powder and a polymeric binder. The polymeric binder may be any suitable polymeric material, or combination of polymeric materials that, when mixed and compressed with the selected metallic powder forms a projectile with a requisite hardness, durability, and importantly, the desired density. Examples of suitable polymeric binders include thermoset and thermoplastic resins, which are actuated or cross-linked by heating. Examples of suitable thermoset resins are melamine and powder-coating epoxies. Examples of suitable thermoplastic resins are nylon (including nylon 6), polyethylene, polyethylene glycol and polyvinyl alcohol. Other suitable polymeric binders are water-actuated polymers, such as Portland cement, vinyl cement and urea formaldehyde, which are cross-linked by immersion or other contact with water. Still another example of a suitable polymeric binder is a pressure-actuated polymer, such as gum arabic. Still further examples of polymeric binders that may be used are gelatin powder and stearic acid.
 Particularly well-suited polymeric binders are elastomeric, or flexible, epoxies, which are thermoset resins that are suitable for use as corrosion-resistant coatings on rebar. Because rebar is often bent after being coated, its coating must bend with the rebar to provide the intended corrosion resistance. As such, these epoxies are often referred to as “rebar epoxies.” Through experimentation, it has been discovered that these epoxies are particularly well-suited for use as a polymeric binder for forming the desired bullet core. Examples of suitable elastomeric epoxies for use as binder are sold by the 3M Corporation under the tradename 3M 413™ and by the Dupont Corporation under the trade name 2-2709™. It should be understood that other elastomeric or flexible epoxies may be used and are also within the scope of the invention.
 When present, the polymeric binder will typically comprise in the range of approximately 0.1% and approximately 10% of bullet core 12, and preferably is present in the range of approximately 0.2% and approximately 3%. An example of a subset of this range is approximately 0.25% and approximately 0.65%. The amount of polymeric binder used is typically rather small, because such binders tend to have much lower densities than the metallic powders. However, the careful selection of binder composition, metallic powder composition, and their relative amounts permit the formation of bullet cores having a wide range of desired densities. Illustrative, non-exclusive examples of proportions of binders that have proven effective include 1-2% melamine, 1.5-5% Portland or vinyl cement, 2-3% urea formaldehyde, and 2-3% gum arabic, although other percentages of these binders may be used and are within the scope of the present invention. It is also within the scope of the invention that the binder may include two or more different types of polymeric binders. For example, a combination of a rigid epoxy and a flexible epoxy may be used to produce a bullet that has increased strength over a comparable bullet formed with only a rigid epoxy or only a flexible epoxy. When more than one binder is used, it is preferable that the binders are cured through the same or compatible mechanisms.
 Non-exclusive examples of suitable bullet compositions include the following: 100 g of WHA/Fe (73.64% WHA/256.36% Fe), 161 g of WHA, 4-8 g binder; 50 g WHA/Fe (73.64% WHA/256.36% Fe), 80.5 g WHA, 4 g 3M431 and 0.27 g lubricant; 65.25 g WHA, 65.25 FeW (73.64% WHA/256.36% Fe), 4 g 3M431 and 0.27 g lubricant; 130.5 g FeW, 3.5 g 3M431 and 0.27 g lubricant; and 116.5 g FeW, 14 g Fe, 2.4 g 3M431 and 0.27 g lubricant. As discussed, ACRAWAX™ is a suitable lubricant.
 To form a bullet core using a metallic powder and a polymeric binder, one or more metallic powders are mixed with one or more polymeric binders. The resulting mixture is placed within a die or other suitable mold that defines at least substantially the shape of the desired bullet core and provides a base or frame against which the powders and binders may be compressed. The mixed powder(s) and binder(s) are then compressed via any suitable mechanism. In experiments, 9800 lb. force applied using various compaction ram diameters, such as 0.33 inches, has proven effective. A rotary press system has also proven effective for use in this step. It should be understood that the particular amount of force and method of compression may vary. Similarly, it should be understood that the degree of compression required will tend to vary, such as depending upon the particular densities of the powder(s) and binder(s) and most importantly the desired density of the bullet core produced therefrom. In some embodiments, after the compression step is completed, the bullet core is suitably formed and ready to be jacketed. Alternatively, the produced bullet core may be sintered prior to jacketing and/or assembling the completed cartridge.
 Some polymeric binders will often need to be actuated to achieve the desired cross-linking, curing, setting or adhesion. The particular method of actuating the binder will tend to vary depending upon the particular binder or binders being used. For example, some binders are actuated by heating. Others are actuated by hydration, and still others are actuated by compression. It should be understood that the actuating step may, in some embodiments, occur during the compression step, such as when heat or pressure are used to actuate the binder.
 Examples of heat-actuated binders include thermoplastic resins and thermoset resins, including the above-described rebar epoxies. It has been found that heating a bullet core at a temperature in the range of approximately 150° F. and approximately 450° F. for a time period in the range of 30 seconds and several hours is effective. Some bullet cores may have a greater tendency to crack as they are exposed to higher temperatures for longer periods of time, and therefore it should be understood that the temperature and time period may vary depending upon the particular bullet composition being used. Heating at a temperature less than approximately 250° F. has proven effective, such as at temperatures less than approximately 200° F. and temperatures in the range of approximately 150° F. and approximately 175° F. Similarly, heating for less than approximately 15 minutes has proven effective, with heating for less than approximately 5 minutes being preferred.
 Because the particular composition of the desired bullet core will vary depending on the particular powders and binders being used, and relative concentrations thereof, it should be understood that temperatures outside of this range may be effective for a particular bullet composition. For example, bullets using melamine as a polymeric binder have been effectively cured at temperatures in the range of 340° F. and 410° F. for several minutes without cracking. It should also be noted that the curing rebar epoxies at 150-175° F. for approximately 5 minutes has proven effective when these epoxies are used as the polymeric binder, despite the fact that these epoxies are normally cured at much higher temperatures when used as rebar epoxies.
 Examples of water-actuated binders include Portland cement, vinyl cement and urea formaldehyde. Typically, the actuation step included immersion of the bullet in water, followed by a drying period. In experiments, the bullets were immersed in water from between a few seconds and almost an entire day. For most water-actuated binders, an immersion, or water-contacting, period of less than an hour, and preferably less than a minute and even more preferably approximately 5-10 seconds was sufficient.
 It is also within the scope of the present invention that the bullets are formed from binders that include both a polymeric component and a metallic component. Expressed another way, bullets formed from powders may include a polymeric binder and a metallic binder. An illustrative example of such a binder is tin and an epoxy.
 As another illustrative method for forming the bullet of the invention, the metallic powder and selected polymeric binder are placed into a jacket prior to compressing the powder and binder to form the bullet core. For example, the powder and binder may be mixed and then added to the jacket, which may subsequently be placed into a die. Alternatively, the jacket may be placed into a die or other suitable mold, and then the mixed powder and binder are added thereto. It should be understood that these methods are intended to provide illustrative examples of suitable methods for forming bullets according to the present invention. It should be further understood that other methods may be used.
 Metallic Binders.
 In another aspect of the invention, the binder is a metallic binder. In one aspect of the invention, the binder is a tin-containing powder. The tin-containing powder may be pure or at least substantially pure tin powder. It may also be a powder that contains tin and other elements, such as a powder containing a tin alloy, such as bronze. The weight percentage of tin-containing powder in the desired bullet core may be tailored so as to achieve the desired density of the bullet core. For example, a tin-containing powder may be present in the bullet core in the range of 5 wt % and 60 wt %. In some embodiments, tin will be present in the range of 10 wt % and 50 wt %, in the range of 15 wt % and 40 wt %, and in the range of 20 wt % and 30 wt %. In some embodiments, the bullet core will contain at least 10 wt % of the tin-containing powder, in some embodiments the bullet core will contain less than 50 wt % of tin-containing powder. Tin has a density of 7.3 grams/cm3.
 As discussed above, tungsten-containing materials are available in a variety of forms, from powders of pure tungsten (density 19.3 g/cc), powders of a tungsten alloy, powders of more than one tungsten alloy, and combinations thereof with a wide range of densities. By mixing these powders in an appropriate proportion with less dense materials, such as tin powder, bullet cores with a wide range of densities may be prepared.
 To form the desired bullet core, the metallic powder and the tin-containing binder are mixed via any suitable mechanism, such as those conventionally used to mix metal powders. Examples of suitable devices include a blender, such as a V-Cone blender, and a grinding mill. The amount of each powder is selected based primarily on the desired density of the finished article, based upon the force with which the powder will be compacted and the densities of the powders. For example, when a tungsten-containing powder contains ferrotungsten powder and WHA powder, less of the tungsten-containing powder will be required to obtain the same density as a corresponding article made without WHA powder. The powders may optionally be mixed in a high-energy mill or attritor to obtain mechanical alloying effects, such as described in U.S. Pat. No. 6,248,150 (supra).
 Similar to the bullet core manufacture procedure described above for use with polymeric binders, the mixed powders are placed into a die and then compressed to form the desired bullet core. The compaction pressure may vary, depending, for example, upon the size, shape and intended use and density of the finished article. At least in the context of firearms projectiles, compaction pressures in the range of approximately 48 to approximately 82 ksi (48,379-82,245 psi) have proven effective, and in some embodiments compaction pressures of greater than 40 ksi, greater than 60 ksi, greater than 80 ksi and even greater than 100 ksi have proven effective. Another method for forming the desired bullet core from the selected metallic powders is through metal injection molding, in which the powders are mixed with a lubricant, extruded and then sintered.
 It should be understood, however, that these pressure ranges are presented for purposes of illustration and that pressures outside of these ranges may be used and are within the scope of the present invention. In particular, increasing the compaction pressure reduces the voids or free-space within the bullet core, thereby increasing the density of the resulting bullet core. By way of background, all mixtures of powdered components have a theoretical density that can be calculated based on the compositions and weight percentages of the powders. Typically, an article produced by compacting these powders will not achieve this theoretical density because of voids or free-spaces within the article. As the mixture of powders is compacted at higher pressures, the amount of void space is reduced, or even eliminated. It should be apparent that careful selection of compaction pressure can also be used to produce a bullet core of the desired density for the purposes of the invention.
 The powders may be cold compacted, namely, compacted without heating. Additionally or alternatively, the compacted powders may be heated or annealed and/or sintered, but heating of any type is not required. Typically, frangible articles are not sintered, but they may or may not be heated or annealed. Sintering may be either solid-phase sintering, in which the article is heated to near the melting point of the lowest melting component, or liquid-phase sintering, in which the article is heated to or above the melting point of the lowest melting component.
 Prior to placing the mixed powders into a die, the die may be lubricated to facilitate easier removal of the compacted article from the die. Any suitable die lubricant may be used. Similarly, it is also within the scope of the invention that a lubricant may be mixed with the powders prior to compaction, as discussed above. Generally, the addition of a lubricant to the powders decreases the density of the compacted article. Typically, but not exclusively, non-metal lubricants are only present in less than 2 wt %, and often less than 0.5 wt % (such as 0.2-0.3 wt %).
 However, the tin-containing powder not only binds the tungsten-containing powder together, but also provides sufficient lubrication. In other words, the bullet core may be produced entirely from metal powders, without requiring the addition of wax, polymers or other lubricants or non-metallic binders. Typically, the tin-containing powder is present in at least 10 wt % to obviate the need for a lubricant. It is also within the scope of the invention that other relatively soft metals, such as copper, may be used as a metallic lubricant and binder.
 The size of the individual particles of the components of bullet core may vary. In the context of at least firearms projectiles, the tin-containing powder having a nominal (average) particle size of 150 mesh has proven effective. Similarly, a tin-containing powder having a nominal size of 80 mesh, with no more than 75% being minus 325 mesh has also proven effective. Suitable tin powder is available from Acupowder, Inc. and sold under the trade name ACU-150™. Similarly, a ferrotungsten powder having a particle size of minus 100 mesh, minus 140 mesh and minus 200 mesh has proven effective, with less than 10-12% being minus 325 mesh being particularly effective. A WHA powder having a size of minus 40 mesh has proven effective.
 It should be understood that these particle sizes are presented for purposes of illustration and not limitation. Similarly, the acceptable particle sizes may vary depending upon the particular mix and composition of powders used to form the bullet core, as well as the particular shape and size of the article to be formed. For example, when the bullet core takes the form of a firearms projectile, it is desirable for the non-compacted mixture of powders to have sufficient flowability (Hall flow test) to readily fill the dies that give the projectiles their shapes. In some embodiments, it may be desirable for the lower density powder(s) to be finer than the higher density powder(s) to discourage separation of the powders after mixing but prior to compaction.
 The following table provides examples of how careful selection of the composition of the bullet core can result in a range of desired bullet core densities. The examples are presented in table-form and are presented to provide illustrative, non-limiting examples of articles that may be produced according to the present invention. For example, only ferrotungsten, WHA and tin powders are shown in the table, but it is within the scope of the invention that other tungsten-containing powders, including pure tungsten and tungsten carbide, and tin-containing powders may be used to achieve the desired ballistic properties. Similarly, it is within the scope of the invention that the bullet core of the invention may include additional components as well, such as powders of other metals or metal alloys. For example, iron powder may be added to reduce the density of the article.
 Bullet Jackets
 As discussed above, the jacket for the bullet of the invention is optionally a metal jacket or a non-metal jacket. Where the jacket includes metal, the jacket material may include copper, or alloys of copper, such as brass. Alternatively the metal jacket may include a ferrous metal alloy, or aluminum. Where the bullet jacket is a metal jacket, the bullet may be formed by compressing the powder and binder to form the bullet core in the jacket itself. Alternatively, the bullet core may be formed, placed in the jacket, and the jacket then crimped around the bullet. In another alternative, the bullet core may be formed, and the jacket subsequently applied to the bullet core by electroplating, vapor deposition, spray coating or other methods. Where the bullet jacket is a non-metal jacket, it is typically a polymer or plastic, such as nylon. The non-metal jacket may be applied by dip coating, spray coating, or any other suitable application method.
 The bullet jacket at least substantially encloses the bullet core. In one aspect of the invention, the bullet core is entirely enclosed. In another aspect of the invention, the bullet core encloses substantially all of the bullet core, leaving the tip of the bullet unjacketed. A variety of jacket configurations may be envisaged by one of skill in the art.
 Although the present invention has been shown and described with reference to the foregoing operational principles and preferred embodiments, it will be apparent to those skilled in the art that various changes in form and detail may be made without departing from the spirit and scope of the invention. The present invention is intended to embrace all such alternatives, modifications and variances which fall within the scope of the appended claims.
FIG. 1 is a schematic cross-sectional view of a bullet constructed according to the present invention.
FIG. 2 is a schematic cross-sectional view of a firearms cartridge containing the bullet of FIG. 1.
FIG. 3 is a comparison of schematic cross-sectional views of the bullet of FIG. 1, a jacketed reference bullet, a thinly jacketed reference bullet, and a non-jacketed reference bullet.
FIG. 4 is a plot illustrating an example of the relationship between bullet caliber and bullet core density when the jacket thickness is fixed.
 The present invention relates generally to the field of firearms projectiles, and more particularly to an improved jacketed bullet, projectile, and method for preparing the same.
 Conventionally, most firearms projectiles are formed from lead. However, lead projectiles can represent a significant health hazard. Lead itself is toxic, and the repeated firing of lead projectiles in enclosed firing ranges can result in lead poisoning through the inhalation and ingestion of lead dust. Lead is also a significant environmental contaminant. For example, in 1996 the Environmental Protection Agency banned the use of lead shotgun shot for hunting waterfowl. In addition to toxicity issues, the use of lead projectiles can result in an increased risk of injury due to the potential for projectile fragments to ricochet from target surfaces. This risk may be enhanced during close-range target practice, where the firearm operator is nearer the target and so the risk of a high-energy ricochet is greater.
 Frangible bullets, or bullets that disintegrate upon impact, have been manufactured, as have various lead-substitute bullets. However, these bullets fail to exhibit the ballistic characteristics of a conventional lead bullet, thereby reducing their utility for target practice or actual use. In addition, jacketed versions of such bullets can continue to present a ricochet hazard, as the jacket is typically substantial enough to fragment and rebound from a target. What is needed is a bullet that minimizes the known hazards of lead toxicity and/or fragment ricochet, while still exhibiting at least substantially the same ballistic characteristics as a reference bullet, such as a conventional lead bullet.
 The present invention is directed to a jacketed firearms projectile having a jacket thickness less than approximately 0.025 inches thick, where the density of the projectile core has been selected so that the jacketed projectile has a weight that is at least substantially the same as a reference firearms projectile, such as a lead projectile, having substantially the same size and shape. In some embodiments, the projectile is a frangible projectile. In some embodiments, the projectile is lead-free. In some embodiments, the projectile is incorporated in a cartridge. Various methods for forming the projectile and forming cartridges containing the projectiles are also disclosed.