I'm pretty sure that the amount of ferro-magnetic material that composes a projectile such as that would be very little, but I'll give Magneto the benefit of the doubt (he lifted a bridge, I think he can stop a bullet).
Anyways, if he is calm and unaware, he's dead. If he has any way to expect it, the sniper is dead.
Anyways, if he is calm and unaware, he's dead. If he has any way to expect it, the sniper is dead.