Preferably, you would need a large, stable bullet with a muzzle velocity of at least 2,700 to 2,800 fps. The best of all worlds would be the .50 BMG comin' out of one of these ...
... but, make it a good shot ... 'cuz you don't want one of these 650 to 800 grain bad boys cuttin' its way through hotel rooms below, beside, or above the gunman's room.
The essential American soul is hard, isolate, stoic, and a killer. It has never yet melted. ~ D.H. Lawrence