Since the operator is completely exposed, it is logical to kill the gunner and not attempt to destroy the gun itself. The gunners are usually positioned low to the ground and as such can be difficult targets to hit. It is advised to attempt this from cover, since the rapid rate of fire poses a serious challenge. Once the operator is down, a well placed grenade or other explosive should be used to destroy the cannon. This is to prevent other Grunts from operating the cannon, which is common after the original operator has been killed. Plasma Cannons are often mounted on Covenant watchtowers for a superior vantage point. Facing them in such situations, distance and/or firepower is key, and rushing such emplacements is not recommended on higher difficulties.
Regardless of how many rounds have been fired when it was stationary, when it is detached, the ammunition is always capped at 200.