M40 HEDP grenade (Earth-7149)

"Lieutenant, what do the pulse rifles fire? 10 millimeter explosive-tip caseless. Standard light armor-piercing round, why?"

- - Ellen Ripley and Lieutenant Scott Gorman

The 10×24mm caseless, designated the M309 in the service of the UNSC Marine Corps, is a caseless rifle cartridge that is used as the primary ammunition for the M41A pulse rifle and its variants.

Overview
The cartridge consists of a 210 grain (13.6 gram), steel-jacketed and explosive-tipped projectile embedded within a rectangular propellant block of Nitramine 50. Although the propellant mass is small, it is highly efficient, generating muzzle velocities of 840 meters per second. The M309 is optimized for lethality against infantry wearing personal armor, with its factory pre-set impact fuse designed to explode after the round has penetrated armor to inflict maximum damage. Consequently, the round is less effective against unarmored opponents, as it will often over penetrate soft targets without detonating, leaving the victim relatively unscathed and necessitating the use of multiple hits to score a kill.

10x24mm firearms

 * M41A Pulse Rifle
 * M41A/2 Pulse Rifle

Trivia

 * Source.