According to the results of the Second World War, it was concluded that it is necessary to improve the design of the most expendable type of ammunition (ammunition for automatic small arms) and expand the resource base for their production.
Cartridges with metal sleeves
The saturation of infantry units with automatic weapons in the defense industry caused a shortage of copper, traditionally used in cartridge brass (used for the production of cartridge cases) and tombac (used for the production of bullet shells).
The most effective solution to the problem of resource shortages was the use of mild steel, coated on both sides with copper to protect against corrosion, or without coating, used in wartime to produce so-called surrogate shells. In the post-war period, the technology of coating steel liners with a special lacquer was mastered, protecting them from moisture and reducing friction in the chamber (up to a certain temperature limit).
Despite the similarity of the technical characteristics of mild steel and copper alloys, the latter have an advantage in ductility and corrosion resistance. Lacquer coating of steel sleeves has low wear resistance and in the process of reloading when in contact with the metal parts of the weapon has the ability to be damaged and transferred to the elements of automation, disabling them. In the case of the extraction of unused cartridges from the barrel after the end of firing, their liners lose their lacquer coating due to its burning out when they come into contact with the heated surface of the chamber, after which they are rapidly oxidized and the cartridges become unsuitable for further use.
The increased consumption of ammunition by infantrymen armed with automatic weapons was the basis for increasing the wearable ammunition by reducing the weight of ammunition. Until the beginning of the 1970-ies, the main direction of reducing the weight of the wearable ammunition was first to switch to intermediate, and then to low-impulse cartridges, due to the desire to increase the accuracy of automatic fire from uncomfortable positions. After adopting the AK-74 assault rifle and the M-16 automatic rifle, this reserve for reducing the weight of wearable ammunition was exhausted - an attempt to use lighter arrow-shaped bullets revealed their increased wind drift.
At present, bullets with a steel core, a lead shirt and a tompak shell are used predominantly as striking elements. In order to increase armor penetration, the US Army switched to using M80A1 EPR and M855A1 all-metal bullets without a lead shirt consisting of a tompac shell and a core with a steel head and a tail of bismuth.
In the 1980-ies in the USSR and NATO countries, an attempt was made to radically solve the problem of the high material consumption of classical cartridges by switching to sleeveless ammunition. The greatest progress in this direction was made by the German company Heckler und Koch, which created the HK G11 automatic rifle, using the DM11 cartridgeless cartridges developed by Dynamit Nobel.
However, the military operation of the 1000 series of HK G11 rifles in the Federal Border Service of Germany showed their danger to servicemen due to the regular spontaneous combustion of caseless cartridges in the chamber, despite its structural separation from the rifle barrel. As a result, the German border guards were first banned from using automatic firing mode, and then generally removed HK G11 from service because of the senselessness of its use as a purely self-loading weapon in the presence of overly sophisticated automation (“cuckoo clock”).
Cartridges with plastic sleeves
The next attempt to reduce the material consumption of small arms ammunition and increase wearable ammunition was carried out in 2000 in the USA by AAI (currently Textron Systems, Textron Production Division) under the LSAT (Lightweight Small Arms Technologies) program, which led to the creation of a light machine gun and an automatic carbine, designed for combined ammunition with cartridges with a brass sleeve, plastic sleeve and caseless, made in a telescopic form factor.
The sleeveless cartridges were expected to spontaneously ignited in the barrel chamber, despite its detachable design, therefore the choice in the LSAT program was made in favor of cartridges with a plastic sleeve. However, the desire to reduce the cost of ammunition led to the wrong choice of type of plastic: as such was used polyamide, which has all the necessary characteristics, except for one, but the most important - its maximum operating temperature does not exceed 250 degrees Celsius.
Back in 1950-s, it was determined by the results of field tests that the barrel of a DP machine gun in the conditions of continuous firing with bursts with interruptions for changing stores heats up to the following values:
150 shots - 210 ° C
200 shots - 360 ° C
300 shots - 440 ° C
400 shots - 520 ° C
In other words, in conditions of intense combat, after the first two hundred cartridges have been used up, the barrel of the light machine gun is guaranteed to reach the melting point of the polyamide.
In connection with this circumstance, the LSAT program in 2016 was closed and the CTSAS (Cased Telescoped Small Arms Systems) program was launched on its basis in order to develop telescopic cartridges on a new material basis. Judging by the interview with the US Administrator Cory Phillips of the program, given to thefirearmblog.com online edition in March 2017, the most heat-resistant structural polymer, polyimide, whose maximum operating temperature is 400 ° C, was chosen as the material for the plastic sleeves.
Polyimide as a cartridge case material also has another valuable property — when heated above this level, it becomes charred without melting to release volatile substances that do not contaminate the barrel chamber, while the charred surface of the sleeve serves as an excellent antifriction material when it is extracted after firing. The strength of the flange of the sleeve provides a metal flange.
The temperature in 400 degrees is the permissible limit for heating the barrels of small arms, after which their distortion occurs, since the temperature of the technological tempering of barrels ranges from 415 to 430 degrees. However, the tensile strength of polyimide at a temperature of 300 and more degrees drops to 30 MPa, which corresponds to the pressure in the chamber 300 atmospheres, i.e. an order of magnitude less than the maximum pressure level of powder gases in modern models of small arms. When you try to remove the cartridge case from the chamber of the classic design, a metal flange will be torn off with a ramrod being knocked out of the barrel.
Heating of the cartridge in the classic design chamber can be controlled to a certain extent by firing from an open bolt (machine guns), but in the case of intense shooting and firing from a closed bolt (automatic rifles and automatic rifles) heating of the cartridge over 400 degrees is almost inevitable.
Cartridges with aluminum sleeves
Another alternative to copper alloys is aluminum alloys used in the cartridges of the serial pistol cartridges, in the pilot development of rifle cartridges and in serial shots to the 30-mm GAU-8A automatic cannon. Replacing copper with aluminum allows you to remove the limit on the resource base, reduce the cost of the liner, by 25 percent reduce the weight of the ammunition and, consequently, increase the wearable load.
In 1962, at TsNIITOCHMASH, experienced cartridges of caliber 7,62x39 mm with an aluminum alloy sleeve (GA code) were developed. The liner had an anti-friction graphite coating. In order to prevent electrochemical corrosion, the primer cup was made of aluminum alloy.
However, the use of such shells is prevented by their only negative property - self-ignition of aluminum and its alloys in air when heated to 430 ° C. The heat of combustion of aluminum is very high and amounts to 30,8 MJ / kg. The external surface of products is subject to self-ignition when heated to the specified temperature and the permeability of the oxide film to air oxygen increases or when heated to a lower temperature in case of damage to the oxide film. A non-plastic ceramic oxide film (thickness ~ 0,005 μm) is destroyed by the deformation of a plastic metal sleeve under the action of the pressure of powder gases, the permeability of the oxide film is achieved by heating with intense firing. The cartridges self-ignite only in air after extraction from the barrel, where a negative oxygen balance is maintained during the combustion of gunpowder.
Therefore, aluminum sleeves have been distributed only as part of pistol cartridges of 9x18 PM and 9x19 Para calibers, the intensity of firing of which and the temperature reached in the chamber does not match any of these indicators machine guns, automatic rifles and machine guns.
Aluminum was also used in an experienced cartridge 6x45 SAW Long, the sleeve of which was equipped with an elastic silicone liner, which tightens the cracks in the metal and oxide film. However, this decision led to an increase in the linear dimensions of the cartridge, the associated size of the receiver and, accordingly, the weight of the weapon.
Another solution, but brought to service, is the 30-mm artillery shot 30x173 GAU with an aluminum alloy shell. This was made possible through the use of a special low molecular weight "cold" propellant charge. The thermochemical potential of the powder is directly proportional to the combustion temperature and inversely proportional to the molecular weight of the combustion products. The classic nitrocellulose and pyroxylin powders have a molecular weight of 25 and a burning temperature of 3000-3500 K, and the molecular weight of the new powder is equal to 17 at a burning temperature of 2000-2400 К with the same impulse.
Perspective cermet sleeve
The positive experience of the use of artillery shots with an aluminum sleeve makes it possible to consider this metal as a structural material for the shells of small arms cartridges (even without a special throwing composition). In order to confirm the correctness of this choice, it is advisable to compare the characteristics of brass and aluminum alloy liners.
Brass L68 contains in its composition 68 percent copper and 32 percent zinc. Its density is 8,5 g / cm3, hardness - 150 MPa, tensile strength at 20 ° C - 400 MPa, elongation at tensile - 50 percent, coefficient of sliding friction on steel - 0,18, melting point - 938 ° C, temperature zone of brittleness - from 300 to 700 ° C.
As a replacement for brass, it is proposed to use aluminum doped with magnesium, nickel and other chemical elements in a volume fraction of no more than 3% in order to increase the elastic, thermal and foundry properties without affecting the resistance of the alloy against corrosion and cracking under load. The strength of the alloy is achieved by its reinforcement with dispersed fibers of aluminum oxide (diameter ~ 1 μm) in a volume fraction of 20%. Protection against surface self-ignition is provided by replacing the fragile oxide film with ductile copper / brass coating (thickness ~ 5 μm) applied by electrolysis.
The obtained cermet composite belongs to the class of cermets and is formed into the final product by injection molding in order to orient the reinforcing fibers along the axis of the sleeve. The anisotropy of the strength properties makes it possible to preserve the flexibility of the composite material in the radial direction to ensure tight contact of the liner walls with the surface of the chamber under the action of the pressure of powder gases in order to obturation the latter.
Antifriction and extreme pressure properties of the liner are ensured by applying a polyimide-graphite coating (thickness ~ 10 μm) on its outer surface with equal volume fractions of binder and filler that can withstand the 1 GPa contact load and the operating temperature of the ICE pistons.
The density of the cermet is 3,2 g / cm3, tensile strength in the axial direction: at 20 ° C - 1250 MPa, at 400 ° C - 410 MPa, and tensile strength in the radial direction: at 20 ° C - 210 MPa, at 400 ° C - 70 MPa, relative elongation under tension in the axial direction: at 20 ° C - 1,5%, at 400 ° C - 3%, relative elongation at tension in the radial direction: at 20 ° C - 25%, with 400 ° C - 60 % melting point - 1100 ° C.
The slip friction coefficient of the antifriction coating on steel is 0,05 with a contact load of 30 MPa and higher.
The technological process of production of cermet sleeves consists of a smaller number of operations (mixing metal with fiber, casting sleeves, hot rolling of collar and dulce, brassing, anti-friction coating) compared to the number of operations in the manufacturing process of brass sleeves (casting billets, cold drawing in six aisles, cold knurling and dulz).
The weight of the brass sleeve of the cartridge 5,56х45 mm is equal to 5 grams, the weight of the cermet sleeve is 2 gram. The cost of one gram of copper is 0,7 US cent, aluminum - 0,2 US cent, the cost of dispersed aluminum oxide fibers - 1,6 US cent, their weight in the liner does not exceed 0,4 gram.
In connection with the adoption of army bulletproof vests of class 6B45-1 and ESAPI, not punched by handgun bullets with a steel core at a distance of 10 or more meters, it is planned to switch to the use of bullets with a core of sintered alloy of tungsten carbide powders (95%) and cobalt (5%) with a specific gravity of 15 g / cc, not needing to be weighted with lead or bismuth.
The main material of the shell of bullets is tompac consisting of 90% copper and 10% zinc, the density of which is 8,8 g / cc, melting point - 950 ° C, tensile strength - 440 MPa, compressive strength - 520 MPa, hardness - 145 MPa, elongation - 3% and the coefficient of sliding friction on steel - 0,44.
Due to the increase in the initial speed of bullets to 1000 and more meters per second and an increase in the rate of fire to 2000 and more shots per minute (AH-94 and HK G-11), the tompac ceased to meet the requirements for bullet shells due to the large thermoplastic wear bore due to the high coefficient of sliding friction of the copper alloy on steel. On the other hand, artillery shells are known, in the construction of which the copper lead belts are replaced by plastic (polyester), the friction coefficient of which is at the level of 0,1. However, the operating temperature of the plastic belts does not exceed 200 ° C, which is half the maximum temperature of the trunks of small arms before they begin to warp.
Therefore, it is proposed to use a polymer composite (thickness ~ 0,5 mm) as an envelope for a promising bullet with an all-metal core, containing in equal volume fractions of the PM-69 type polyamide and colloidal graphite with a total density of 1,5 MPa, tensile strength 90 MPa compression, 230 MPa hardness, 330 MPa contact load, maximum operating temperature 350 ° C, and sliding friction coefficient on 400 steel.
The shell is formed by mixing the polyimide oligomer and graphite particles, extruding the mixture into a mold with embedded parts - the core of the bullet and temperature polymerization of the mixture. The adhesion of the shell and the core of the bullet is ensured by the penetration of the polyimide into the porous surface of the core under the action of pressure and temperature.
Perspective telescopic cartridge
Currently, the most progressive form factor of a small arms cartridge is considered to be telescopic with the placement of a bullet inside a pressed projectile charge. The use of dense checkers instead of the classic granular charge with a lower bulk density allows one and a half times to reduce the length of the cartridge and the associated envelope of the receiver of the weapon.
Due to the design features of the reloading mechanism (detachable barrel chamber) of small arms models (G11 and LSAT) using telescopic cartridges, their bullets are recessed into the thrusts of the propelling charge below the edges of the sleeve. The open end of the secondary propellant charge from dirt and moisture protects the plastic cap, which also plays the role of the front obturator when fired (by blocking the joint of the detachable chamber and the barrel after the bullet breaks through). As the practice of troop exploitation of DM11 telescopic cartridges has shown, this method of cartridge configuration, which does not ensure that the bullet stops in the bullet entrance of the barrel, leads to bullet distortions when fired and, accordingly, loss of accuracy.
To ensure the specified sequence of operation of the telescopic cartridge, its propelling charge is divided into two parts - the primary charge of relatively low density (with a higher burning rate), located directly between the capsule and the bottom of the bullet, and the secondary charge of a relatively higher density (with a lower burning rate), located concentrically around the bullet. After the capsule is pricked, the primary charge is first triggered, pushing a bullet into the barrel bore and creating a boost pressure for the secondary charge that moves the bullet in the barrel bore.
To hold the secondary charge checkers inside the cartridge, the edges of the open end of the liner are partially rolled. The bullet is held in the cartridge by pressing it into the secondary charge checker. Placing the bullet along the entire length in the dimensions of the sleeve reduces the length of the cartridge, but at the same time creates an unfilled volume of the sleeve around the ogival part of the bullet, which leads to an increase in the diameter of the cartridge.
In order to eliminate these shortcomings, a new layout of the telescopic cartridge, designed for use in small arms with a classic non-detachable barrel chamber with any type of reloading mechanism (manual, gas engine, moving barrel, semi-free bolt, etc.) and method of firing (with front or rear sear).
The proposed cartridge is equipped with a bullet that extends beyond the liner with its lively part and thereby rests against the barrel entrance of the bullet. Instead of a plastic cap, the open end of the propellant is protected by a moisture-resistant varnish that burns when fired. Some increase in the length of the proposed cartridge compared with the known telescopic cartridges is compensated by a decrease in its diameter due to the elimination of unfilled volumes inside the liner.
In general, the proposed telescopic cartridge will increase the number of cartridges in infantry infantry ammunition by a quarter, and will also reduce the consumption of materials, laboriousness and cost of production of cartridges.