The plasma rifle fires bolts of superheated plasma and serves as an assault rifle for the Covenant.  There are differences between the plasma rifle in Halo and Halo 2.  In the original Halo, it takes three plasma rifle shots to drain the shields of a Spartan's battle suit,  whereas in Halo 2 it takes 6.5 shots.  This results in 46.67 to 105 kilojoules per bolt in Halo to 21.54 to 48.46 kilojoules per bolt in Halo 2.