Self-contained metallic cartridges haven't really changed a great deal since the introduction of the 8mm Lebel and smokeless powder in 1886. Sure, we have seen certain refinements, such as the introduction of rimless cases, non-corrosive primers, intermediate-length cartridges and small-caliber high-velocity cartridges. Even so, a rifle cartridge from today would not look at all out of place if transported back in time 100 years.
This might be changing in the near future. Currently, there is a great deal of interest in cased telescoped ammunition, and it's possible this just might replace conventional metallic cartridges. As its name suggests, telescoped ammunition is designed so the projectile is partially or completely enveloped by the propellant charge. While telescoped ammunition can be caseless, the U.S. military is currently testing cased designs.
The sample I've seen features a translucent straight-wall polymer case with a centrally located primer in its base. The primer appears to be a conventional Boxer type. The projectile is surrounded by propellant up to its cannelure. The case is open at the front with the projectile nose visible. A single groove runs around the circumference of the forward part of the case.
One large hurdle during the initial development was selecting a suitable polymer for producing the cases. It had to withstand not only the extreme temperature range it might be employed in—minus 50 to plus 150 degrees—but also the extremes of the ballistic cycle. Out of dozens of polymer types considered only one high-quality medical grade polymer proved suitable.
It's also interesting to note both loose and compacted propellants were tried. Researchers found compacted propellant provided better burn characteristics and in turn led to a slight increase in accuracy. It also required less case volume, which made for a slightly smaller cartridge.
The polymer cartridge case consists of two parts: the body and the end cap. During loading the body is charged with propellant. The end cap, which has the projectile attached directly to it, then seals off the front of the case. The end cap is a robust piece with the projectile held by a slightly undersize sleeve.
Upon firing, the bullet rides through this sleeve, which guides it into the throat of the barrel. This prevents the bullet from becoming slightly off-axis to the rifling, such as can happen with a conventional metallic cartridge damaged during the feed cycle. It is believed this aids accuracy compared to conventional ammunition.
The most obvious advantage of this design is it dispenses with the need for brass or steel cartridge cases, which reduces the manufacturing costs. It also noticeably reduces the weight of the ammunition. It is claimed to be about a 40 percent reduction, which is significant. This allows a soldier to carry more ammunition without carrying more weight.
Overall cartridge length is also reduced with this ammunition, and that in turn can mean a shorter receiver—resulting in a shorter, lighter firearm. Cased telescoped ammunition also requires a simpler operating system, which also leads to a reduction in both weight and complexity—the latter bringing the potential of increased reliability. For one thing, the system eliminates rim problems during the extraction cycle encountered with metallic cartridges.
Plus, because the projectile is seated inside the polymer case, the chance of damaging or setting back the projectile during the feeding cycle is eliminated. And unlike a conventional design, weapon systems utilizing cased telescoped ammunition feed and extract/eject in the same direction. The chamber is actually separate from the barrel, and the new cartridge simply pushes out the spent case during the feeding cycle.
Due to this, there are no timing issues or malfunctions as encountered with a conventional operating system. Instead, everything moves in the same direction. A side benefit to this is the chamber, since it's not attached to the barrel, never gets hot enough to cause a cook off.
While the military has been testing a rising chamber design in an experimental carbine, other types are possible. These include rotating, translating and revolving designs. With an independent chamber separate from the barrel, one wonders how the gas is sealed within the system. The answer is the cartridge itself seals the breech, with the polymer from the case flowing into the empty space to form a seal and obturate the chamber. The difficult part in the design is to get it to seal properly, but without running into issues with removing the fired case.
The Joint Service Small Arms Program initiated an LSAT (Lightweight Small Arms Technologies) light machine gun program in 2004, and caseless ammunition was part of this effort. The result was a weapon that was lighter in weight and more reliable—85,000 rounds had been fired through 10 samples by May 2015. Work on the concept has since expanded into additional calibers and carbines.
What the future will bring I cannot say. The U.S. military is well known for starting elaborate small arms projects that bear no fruit. Cased telescoped ammunition might be different, though. We'll just have to wait and see.