M634 Experimental High-Powered Semi-Armor-Piercing

9.5 mm x 40 is a type of projectile ammunition with a caliber of 9.5 mm that is used by the UNSC battle rifle.

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 but of smaller diameter (9,5mm compared to 12,7), but the BR does more damage. This can be explained with the longer barrel and thus higher muzzle velocity of the rifle.

It is strange that this 9.5mm cartridge was even created. First of all, this cartridge is nearly 2mm wider than the biggest combat rifle cartridge, so it could be safe to say that the BR would have terrible recoil. There are many theories concerning the cartridge. Future technology could counterbalance this large cartridge, maybe some sort of advanced compensator or inhibitor. There is also the delayed blowback principle, in which in semi-automatic mode the rifles recoil occurs after the bullet leaves the barrel. And the heavy bullets are stored in the bull-pup style, so that helps too.

The most likely reason for the catridge's existence would be to fill the gap between the 7,62mm assault rifle bullet and the 12,7mm (.50 cal) machine gun round; assault rifles may have been seen as too weak while machine guns are too heavy to be moved on foot.