M443 Caseless Full Metal Jacket
From Halopedia, the Halo wiki
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 [1].
Background
This ammunition is classified as "caseless," meaning it does not have a metallic casing surround 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 negates the need to expel brass casings, which also is a major cause of jamming, as mentioned above.
This is important, as a user wielding two M7s at once would not want to have hot brass flying in his face from the eject port of the weapon in his left hand. The projectile itself is "jacketed," or coated, in metal (possibly copper or steel) to aid in penetration of the target. "Caseless" rounds are actually embedded into the block of propellant, reducing their length, allowing more ammo to be stored in a smaller space.
