The preferred weapon of Elites in Halo 1; in fact, no other Covenant species in the first game would use it, although Flood combat forms created from Elites and Human marines would also use them.

This weapon will fire continuously if you hold down the trigger at the expense of accuracy, as with most weapons that can be fired this way in Halo. Like the plasma pistol, it has a non-rechargeable battery. It has no other firing modes and its projectiles do not seek.

Some screenshots have shown Jackals holding plasma rifles in Halo 2.

The plasma rifle can be dual wielded, contrary to what it said earlier in this space; as noted below in several comments, there are screenshots that show it being dual wielded, and Bungie's own Dual Wielding Guide, published today, also confirms that the Plasma Rifle is dual wieldable.

As with other plasma weapons, it has a battery that drains as it is fired. When fired too quickly or too often, the weapon overheats and must cool. A temperature gauge indicates when this is about to happen.

The plasma rifle can be fired in semiautomatic and automatic modes.

There is no further information regarding any possible changes to this weapon for Halo 2; it is assumed to be principally the same.