Obtain the Rifle
You can pick up the blue Elite or red Brute version of the Plasma Rifle by killing off one of the aliens armed with the weapon. There are no differences between the blasters outside of the color. In Halo Reach you can choose to start a multiplayer match with a Plasma Rifle if you are an Elite of the Ranger or Warrior class.
Rifle Stats
A Plasma Rifle is an energy-based weapon that drains an opponent's shields before taking their health. A fully charged Plasma Rifle contains 400 shots, and will overheat if you hold the fire button for too long. You cannot reload this gun, and in every Halo game except Halo 2 the Plasma Rifle cannot be duel-wielded.
Effective Usage
When using a Plasma Rifle, take note of the number of shots displayed. If there are 50 or less plasma bursts in the rifle, bring along a powerful and fully loaded secondary weapon. A Plasma Rifle can easily be used from mid to short range, and is an excellent tool for ambushes. By the time your foe turns to face your blue plasma Avalanche, crack his skull with the Plasma Rifle to finish the fight.