As has been pointed out. Magneto did have the ability to stop the bullet fired at the officer in X Men II. Here's the rub... bullets aren't rockets with motors. Once their forward motion caused by the expansion of burning gunpowder is stopped, they stop. The bullet is obviously still being pushing forward into the officer's forehead after forward motion has been stopped, propelled by some force. The gunpowder has been spent and is no longer a factor as a propellant. That force then must be Magneto, unless I'm missing something.
Note: this question is not about who fired the gun or who initially STOPPED the bullet from hitting the cop. It's about the bullet staying at the officer's head and apparently boring forward AFTER its been stopped. Is Magneto trying to push it forward and Jean trying to stop it?
Physics dictates that once the forward momentum of the bullet is stopped, it should just fall to the ground unless it's being HELD in place.