The MA5B asault rifle fires a 7.62x51 millimeter cartridge from a 60 round magazine.  Taking seven rounds to down a Spartan's shields, this means that each bullet has 20 to 45 kilojoules of kinetic energy.  With a muzzle velocity of 905 meters per second, this puts the mass of each bullet at between 48.8 and 109.9 grams.