the present invention is more particularly described in the following examples that are intended as illustrative only since numerous modifications and variations therein will be apparent to those skilled in the art . as used in the specification and in the claims , “ a ” can mean one or more , depending upon the context in which it is used . the preferred embodiment is now described with reference to the figures , in which like components indicate like parts throughout the figures . as shown in fig2 and 4 , in one embodiment , the invention is a jacketed frangible bullet 20 having two main components , an elongated frangible core member 30 and an outer jacket 40 . the embodiment pictured in fig2 and 4 represents a 5 . 56 mm jacketed frangible bullet 20 designed for rifle use , but it should be understood that the scope of this invention extends to all calibers of bullets for use in any pistol or manual , semi - automatic or fully - automatic rifle or machine gun , or any other weapon system in which use of a frangible bullet 20 is advantageous . referring to fig3 the core member 30 has a back end 32 , an opposite tip end 34 and a central portion 36 therebetween . the length of the core member 30 , or the core length l 1 , is defined by the distance between the back end 32 and the tip end 34 . in the embodiment illustrated in fig3 and 4 , corresponding to a 5 . 56 mm rifle bullet , the core length l 1 is approximately 0 . 8 inch . the core member 30 is fabricated from materials including a thermoplastic polymer and at least one metal . both type 11 and type 6 nylon materials ( commonly known in the art as n11 and n6 nylons , respectively ), have been successfully utilized in fabricating frangible munitions . it has been discovered that n6 nylon is preferable to n11 nylon because of its lower cost and higher specific gravity and because of its greater frangibility . n11 nylon , however , exhibits greater shear strength and bonding characteristics over n6 nylon , such that use of n11 nylon permits the use of an increased ratio of metal powder to thermoplastic polymer in the core member 30 composition . in prior art unjacketed frangible bullet designs for 5 . 56 mm weapons , as shown in fig1 bullet performance criteria have not been met by bullets utilizing the less expensive n6 nylon material . thus , prior art 5 . 56 mm unjacketed frangible bullets have been manufactured using n11 nylon materials . as discussed in detail below , however , by the addition of an outer jacket 40 to the frangible bullet 20 , dynamic strength characteristics of the bullet 20 are significantly improved , permitting the use of the less costly and lower shear strength n6 nylon material as the bonding agent for the compound . use of the n6 nylon results in a core material more frangible than compound made with the n11 nylon . according to the invention , other thermoplastic polymer materials in addition to those discussed above may also be utilized , including other polyamids ( including other nylon formulations ), polyesters and polyurethanes . additionally , the thermoplastic polymer component may be fabricated partially or exclusively of polyvinyl chloride , fluorocarbons , linear polyethylene , polyurethane prepolymer , polystyrene , polypropylene and cellulosic and acrylic resins . it is critical , however , that the selected thermoplastic polymer utilized to form the core member 30 be highly frangible in order not to increase the terminal penetration effects of the bullet 20 . a metal component is provided in combination with the above - described thermoplastic polymer to form the core member 30 . the metal component of the core member 30 includes at least one metal , and may include a combination of two or more metals as needed to achieve the desired bullet weight for the selected weapon system . metal components are selected , in part , based upon their density and their suitability for combination with the selected thermoplastic polymer to form a completed core member 30 . because of their relatively high specific gravity , heavy metal elements have been utilized to form the metal component of the core member 30 . the metal component is commonly utilized in powder form , because of the relative ease with which powdered metals may be mixed and combined with the thermoplastic polymer material . alternatively , metals in particulate or other forms may be utilized if they are capable of combination with the thermoplastic polymer . in one embodiment , copper powder and tungsten powder are have been utilized to form the metal component of the core member 30 . in a currently preferred embodiment , a core member 30 including n6 nylon , copper powder and tungsten powder is utilized , in relative concentrations that result in an overall core member 30 specific gravity of 6 . 0 . this material is commercially available from saracen chemicals , ltd ., a united kingdom company with offices at vulcan house , restmer way , hackbridge , surrey , sm6 7ah , england . this material has been found to exhibit superior frangibility over a similar material containing n11 nylon , copper powder and tungsten powder having a specific gravity of 5 . 4 . the addition of the outer jacket 40 , which significantly increases the structural and thermal durability of the bullet 20 , enables use of the n6 nylon core member 30 , resulting in reduced cost and superior performance over the prior art . other metals may be utilized in the core member 30 without departing from the scope of the invention . selected or combined heavy metals including transition metals , lanthanides and actinides provide suitable mass to the core member 30 to meet performance criteria of frangible ammunition . because of price , availability and manufacturing concerns , four common metals have been primarily utilized in the art : zinc , tin , copper and tungsten . any metal selected from this group may be utilized to produce frangible ammunition according to the invention . once the constituent materials are selected , the core member 30 may be manufactured according to methods known in the art , including injection molding . [ 0041 ] fig3 illustrates a reduced diameter , 30 - grain frangible bullet core member 30 according to the present invention , compounded of a mixture of n6 nylon , copper powder and tungsten powder . the core diameter is reduced relative to unjacketed bullet core members to allow for application of the outer jacket 40 illustrated in fig2 and 4 and as described in detail below . [ 0042 ] fig2 and 4 illustrate the placement of the outer jacket 40 on the frangible core member 30 . encapsulating the core member 30 in the outer jacket 40 isolates the core member 30 from direct contact with the heated chamber of the weapon , significantly reducing the potential for softening of the material and in - bore catastrophic failure of the bullet 20 . the outer jacket 40 has a closed first end 42 and an opposite opened second end 44 . the jacket length l 2 is the distance between the first end 42 and the second end 44 of the outer jacket 40 . the hollow interior of the outer jacket 40 is referred to herein as the jacket chamber . the outer jacket 40 encloses a portion of the frangible core member 30 , including the back end 32 and a portion of the central portion 36 of the core member 30 , leaving at least the tip end 34 fully exposed . installation of the outer jacket 40 over the back end 32 of the core member 30 with the tip end 34 exposed ensures that the frangible material will impact the target first . the deceleration shock generated by impact radiates rearward , ensuring the effective disintegration of the entire core member 30 . outer jackets 40 fabricated from copper have been utilized and found acceptable for use in the present invention because of the ductility , weight and deformability of copper . other materials may be utilized to form the outer jacket 40 without departing from the scope of the invention . for example , outer jackets 40 may be fabricated from other metals , including mild steel , guilding metal , guilding clad steel or coated tin . alternatively , synthetic thermoplastic polymer materials may be utilized to form the outer jacket 40 , provided that the selected polymer exhibits sufficient rigidity under barrel operating temperatures , without departing from the invention . when assembled , the back end 32 of the core member 30 is inserted into the jacket chamber until the back end 32 is seated adjacent the first end 42 of the outer jacket 40 . using a dial press , a hand press or other methods known in the art , the second end 44 of the outer jacket 40 is crimped or necked - down to fixedly secure the core member 30 within the outer jacket 40 . adding this outer jacket 40 to the 30 - grain core member 30 produces an overall bullet weight of 45 grains . this jacketing procedure increases the 5 . 56 mm bullet weight by 36 percent . in the illustrated embodiment , the dimensions of the core member 30 and the outer jacket 40 are controlled such that the core length l 1 is greater than the jacket length l 2 . consequently , the tip end 34 , or ogive , of the core member 30 is unjacketed and exposed to impact with targets . as discussed above , this condition has been found to promote full frangibility without significantly compromising performance of the munitions . conversely , testing conducted with fully copper - jacketed frangible bullets has resulted in over penetration and significantly limited disintegration of the bullet . additionally , a lubricant may be applied to the exterior surface of the outer jacket 40 to improve weapon performance . suitable lubricants include coatings such as teflon , a variety of oil - based lubricants such as militec , and a variety of powder - based lubricants such as graphite , which are known generally in the art . it should be noted that the use of some lubricants such as teflon is restricted by various legislation currently in place . an operational frangible munitions cartridge 50 is produced by loading the frangible bullet 20 described above into a cartridge case 52 . referring now to fig2 the cartridge case 52 has a base end 54 and an opposite opened mouth end 56 . the cartridge case 52 is generally hollow , defining a chamber therein which is loaded with powder and air to provide a charge . the jacketed frangible bullet 20 is mounted within the opening defined by the mouth end 56 of the cartridge case 52 as is generally known in the art . the jacketed frangible bullet 20 protrudes from the mouth end 56 of the cartridge case 52 , such that the first end 42 and a portion of the outer jacket 40 are contained within the cartridge case 52 . the second end 44 of the outer jacket 40 and the remaining portion of the outer jacket 40 , including the crimped or necked - down portion of the outer jacket 40 and the tip end 34 of the core member 30 , protrude from the mouth end 56 of the cartridge case 52 . a primer ( not shown ) is provided in the base end 54 of the cartridge case 52 that detonates the charge when the weapon is fired . the sudden and substantial pressure created by the combustion of the charge within the confined chamber within the cartridge case 52 impart the forces necessary to eject the frangible bullet 20 from the cartridge case 52 and accelerate the bullet 20 to its firing speed . the diameter of the bullet 20 illustrated in fig4 is full caliber , meaning that the outside dimension of the bullet 20 conforms to the minimum and maximum outside dimensions established for this caliber bullet 20 by the sporting arms and ammunition manufacturers institute ( saami ) specification . the overall length of the cartridge 50 , once loaded in the cartridge case 52 , also falls within those standards . testing of an unjacketed 33 - grain 5 . 56 mm frangible cartridge described above in the discussion of the prior art provided pressure results at the chamber of 124 , 136 pounds per square inch and 9 , 695 pounds per square inch at the gas port . these results indicate that the unjacketed 33 - grain bullet of the prior art was producing case pressures 8 , 000 to 10 , 000 pounds per square inch less than currently used , non - frangible military ball ammunition . more importantly , the gas port pressures for the prior art style frangible bullet were 3 , 000 pounds per square inch lower than the minimal acceptable value for ball ammunition . the government &# 39 ; s report of its technical feasibility test concluded that “[ t ] he ramification of these differences is that weapon function may be degraded across the temperature profile since all 5 . 56 mm weapon systems are gas operated .” in contrast to test results from prior art unjacketed 33 - grain bullet test results , testing of the heavier , copper jacketed 45 - grain 5 . 56 mm bullet according to one embodiment of the present invention demonstrated not only improvement , but pressures and accuracy equal to or surpassing that achieved by conventional military combat ball ammunition . in side - by - side testing , the new 45 - grain bullet produced chamber pressures and gas port pressures of 50 , 300 and 14 , 915 pounds per square inch respectively , while the military &# 39 ; s issued ammunition provided pressures of 47 , 000 and 15 , 139 pounds per square inch . these improved pressures result in enhanced reliability and performance of the frangible cartridge to levels that satisfy the military &# 39 ; s requirement . in addition , accuracy improvements surpassed the performance of military ball ammunition at 100 meters and matched military ball &# 39 ; s performance at 200 meters . equally important , side - by - side comparative testing of the 33 - grain unjacketed bullet and the 45 - grain jacketed bullet demonstrated no measurable penetration or frangibility difference between the two cartridges . during all testing by both the inventors and by the government , no instance of catastrophic failure of the jacketed 45 - grain bullet was experienced at operating temperatures that ranged from 60 to − 29 degrees c . although the present invention has been described with reference to specific details of certain embodiments thereof , it is not intended that such details should be regarded as limitations upon the scope of the invention except as and to the extent that they are included in the accompanying claims .