The M41 LAAG fires 12.7x99 millimeter armor penetrating cartridges at 450 to 550 rounds per minute (Halo game manual).  It takes four rounds to drain the shields of Spartan battle armor, giving each bullet a kinetic energy of 35 to 78.75 kilojoules.  Based on the fact that most modern bullets take up roughly one-third of the length of the entire cartridge and assuming that the bullet is cylindrical for half the length and conical for the other half, the total volume of the bullet would be 2.09x10^-7 meters cubed.  Also assuming that the bullets are composed of mostly lead which has a density of 11340 kilograms per cubic meter, each individual bullet masses 31.6 grams.  This gives a muzzle velocity of 1488 to 2232 meters per second.