M634 Experimental High-Powered Semi-Armor-Piercing

9.5 mm x 40 is a kind of ammunition with a caliber of 9.5 mm.

The only weapon that uses this caliber cartridge is the Battle Rifle, which was introduced in Halo 2 and will be returning in Halo 3. The fact that the Battle Rifle does much more damage than the original Assault Rifle]] could have to do with the larger bullets (Assault Rifle uses the 7.62mm AP-FMJ cartridge).

Interestingly enough, the BR and the weaker cousin of the M6D HE Pistol, the M6C Magnum Sidearm, both use bullets with the same length, yet the BR does much more damage than the Sidearm.