Caseless Ammunition - LSATThe caseless propellant of the weapon is fundamentally based on the
Lightweight Small Arms Technologies (LSAT) program, which was intended at it's core to provide lighter, yet still as or more reliable and capable platforms, for infantry as possible. The caseless propellant combines aspects of this design, with more modernized gunpowder, similar to that used in the
.25-45 Sharp and
M885A1 cartridge. The primary advantage of caseless rounds is their reduced weight, in that removing a brass casing helps to reduce the weight of the overall cartridge. For many cartridges, the brass casing can be nearly half the weight of the overall round. For instance, for a 5.56mm, the bullet is roughly 4.1 grams, the propellant (or gunpowder) is 2 grams, and the brass casing is approximately 6+ grams, which leads to about a 12.3 gram overall cartridge. If just the weight of the round and propellant are considered, than the overall required weight for the bullet to function is only 6.1 grams, which is about half the weight the standard 5.56mm cartridge. This means twice as much ammunition can be carried for the same weight, which has substantial benefits in regards to logistics and sustained combat; on the other hand, heavier less practical ammunition, such as the 7.62mm x 51mm NATO, would be nearly the same weight as the 5.56mm, or 12.4 grams (for 2.7 grams for the powder and 9.7 grams for the bullet), allowing it to reduce the weight of the overall system substantially, or put it to the same levels as the 5.56mm. In addition to this, with less mass of the reciprocating parts, the recoil can be reduced substantially. The primary issue with such cartridges however is their increased heat sensitivity and chamber pressure. Because the brass or casing does not absorb both the heat and expanding force of the propellant, the cases are more susceptible to cook offs, or being detonated by accident via increased heat in the chamber. The goal of caseless weapon's, therefore, is to reduce the heat sensitivity of the cartridges, decrease the pressure, or produce a chamber that can handle the increased pressure or heat. Caseless weapons also need an additional seal to seal off the propellant, to act as a virtual casing, attached to the bolt and firing pin.
Based on the original
G11 program, the LSAT ammunition was intended to replicate the heat insensitive binder developed by Dynamit Nobel, the founder of the nobel foundation, and responsible for the development of shock insensitive explosives, such as dynamite. Premature ignition of ammunition from heat in the chamber, known as cook-off, was a major problem with early prototypes of the G11 where synthetically bound nitrocellulose, formed into blocks, was used. Normally, when a bullet is fed into a chamber its case insulates the propellant from igniting until its impact-sensitive primer is struck by a firing pin or striker. The case aids in insulating the propellant from the heat of the chamber and it takes time for the temperature to rise sufficiently, inside a chambered round, to ignite the propellant. In addition, extracting a hot case removes heat from the system. As a result of doing away with traditional cases, the G11 became a safety hazard and had to be withdrawn from the 1979 NATO trials. The high rate of fire and lack of cartridge cases made cooking-off a significant problem since the heat buildup in the G11 chamber was immense, due to the chamber having no provision for cooling, as with a reciprocating bolt system which allows hot air to leave the chamber when the bolt is retracted and the chamber is exposed to air. The vertically swivelling chamber furthermore made gas sealing at each end at such high pressures impractical, as opposed to a cross-sectional round-inside-round bolt-to-chamber fit with appropriate gas sealing . To solve this, Heckler and Koch formed a partnership with Dynamit Nobel who redesigned the cartridge to use a new High Ignition Temperature Propellant (HITP). The cook-off problem was reduced, by using a denatured HMX propellant with a special binder and coating for the ammunition that increased the spontaneous ignition temperature by another 100 °C above that of standard, nitrocellulose (180 °C) propellant.
While the G36 was eventually chosen over the G11 due to it's reduced complexity and cost, future designers of the LSAT sought to replicate this heat insensitive binder. The Heckler & Koch G11 was the only weapon to achieve a service capable assault rifle firing caseless ammunition. Its unique ammunition, designed by Dynamit Nobel, introduced several important innovations, such as improved internal ballistics through the use of a primer, and the prevention of cooking off (the lack of a case makes it easier for a hot chamber to ignite the exposed propellant) through the use of the less sensitive hexogen/octogen as the explosive component. The Advanced Combat Rifle experimental program gave the US Army access to the ammunition and entrenched the ammunition as a viable option. With the high efficiency and lethality of the ammunition, the vast expenditure such a concept had needed for development, and the reduced risk of using an already proven ammunition design, the LSAT program chose a licensed version of Dynamit Nobel's caseless ammunition as a route towards its goal of weight reduction. The LSAT program also uses the same concept of a rotating chamber as the G11 (albeit, the LSAT LMG chamber swings around a longitudinal pivot, whereas the G11 chamber rotated around a horizontal axis at its very centre), in some of it's weapon designs. The High Ignition Temperature Propellant (HITP) was created by Nobel by using a denatured HMX propellant with a special binder and coating for the ammunition that increased the spontaneous ignition temperature by another 100 °C above that of standard, nitrocellulose (180 °C) propellant, which made it far less susceptible to cook offs.
While the program intends to expand into a variety of other cartridges, notably one which mimics the 6.5mm Creedmoor, fundamentally this type of ammunition can be applied to nearly type of cartridge. All that is required is to combine the bullet with the newer propellant and binder, and adapt the gun to possess the seal necessary for firing each round. This makes it applicable to a wide variety of potential firearms, including pistols, rifles, and machine guns. The 5.56mm and 7.62mm are already in development, while plans for future cartridges exist, as well.
Improved gunpowderOn top of using caseless propellant, the weapon utilizes gunpowder which is more powerful, has better thermal stability, a more equalized pressure, provides a more consistent burn (for better accuracy and general performance), and burns more quickly in the barrel, giving it more power from a shorter barrel. The gunpowder is more barrel, and roughly on par with the .25-45 Sharp. Despite using the same amount of gunpowder as the 5.56mm, the more modernized improved gunpowder increases the power from 1800 joules to 2,350 joules per 28.5 grains, or roughly 2 grams of gunpowder. This not only increases the power of the cartridge, but does so with little to no increase in pressure, and is more thermally stable, making it less likely to have variances in unsafe pressure margins. Guns rated to fire the 5.56mm for example must have a maximum overpressure capability of 75,000 PSI, despite the cartridge having an average PSI of 55,000 PSI. This is due to the rare potential for a cartridge that may be unusually powerful to be accidentally produced in any given lot of ammunition, or given how it may behave under certain environmental conditions (such as in the extreme heat or cold). With more thermally stable gunpowder this is less of an issue, and thus increases the safety, as well as the general performance of the round in different environments. The more consistent burn behavior results in higher accuracy, reliability and less variance between each shot fired.
Finally, the gunpowder burns faster in the barrel, thus reducing the need for an extended barrel. A carbine length barrel, like in the M4 carbine, traditionally fires a 4.1 gram projectile approximately 850 m/s, which limits the effective range of the M4 carbine, which only tumbles and fragments consistently at velocities above 760 m/s, to 150 meters, from 300 meters from a standard full length 20 inch long barrel. The shorter barrel length not only decreases the accuracy and power of the cartridge, but exponentially reduces it's effectiveness as below certain velocities many cartridges stopping power and performance falls short of what's desired. By utilizing faster burning propellants however, like those used in the M855A1 Enhanced Performance round, this problem is almost eliminated. The M855A1 Enhanced Performance round achieves approximately 910 m/s from a 14.5 inch barrel, for approximately 1,700 joules of energy, vs. a standard M855 round from a 20 inch barrel with approximately 940 m/s and 1,800 joules of energy. From a full length 20 inch barrel, the M855A1 has approximately 970 m/s, or 1,900 joules of energy, giving it even better performance. This makes the M855A1 nearly as good in the M4 carbine as the M16, reducing the need for extremely long barrels which limit the maneuverability of the weapon, especially in close quarters. While the average pressure is slightly higher at around 62,000 PSI compared to 55,000 PSI for a standard M855, being more thermally stable the maximum pressure is lower than the standard M855.
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