The 5x23mm M443 Caseless Full Metal Jacket (abbreviated M443 Caseless FMJ) is a type of .197-caliber projectile ammunition used by the United Nations Space Command in their M7 Caseless Submachine Gun.
This ammunition is classified as "caseless", meaning it does not have a metallic casing surrounding the powder and sealing the bullet, propellant, and primer together. Instead of using this casing, the round uses a combustible adhesive to seal these components together. Because the chemical "casing" is vaporized when fired, there is no need to eject spent brass shell casings.
This feature is very beneficial. It reduces friction inside the magazine, one of the main causes of jamming in weapons. It also eliminates the need to expel brass casings, which is also a major cause of jamming, as mentioned above.
This feature is very helpful to a user wielding two M7s at once (such as a SPARTAN) who would not want to have hot brass flying in his or her face from the eject port of the weapon in his left hand. The projectile itself is "jacketed," or coated, in a gliding metal (possibly copper or brass) and, to aid in penetration of the target, a hardened steel or Tungsten carbide core. "Caseless" rounds are actually embedded into the block of propellant, reducing their length, allowing more to be stored in a smaller space.
- Real caseless weapons suffer from problems with "cooking off", which is when incredible temperatures result in a cartridge firing unintentionally. Since there is nothing insulating the combustible material from the heat (one of the benefits of a metal casing), even temperatures in the chamber created by firing a few short bursts could result in the cartridge going off, leading to an accidental discharge, damage to the weapon, and possibly even injury the user. The gun may fire from an open bolt, which helps prevents a cook-off by keeping the round out of the hot chamber until the trigger is pulled, as used in the Mac-10 (which may explain the lack of accuracy, as open-bolt weapons are inaccurate), or it may use a High Ignition Temperature Propellant similar to the prototype G11 rifle.