I have a reworked Crosman 2240 .22 caliber CO2 pistol that will put a pellet through a 7/8" pine board up close and blows through both sides of a dog food can from 30 yards. It kills rabbits and squirrels easily. I wouldn't want to rely on it for defense but I wouldn't want to get shot with it either. It'd definitely do more than raise a welt.
Next time you shoot a deer, bring the 2240 along and pop in behind the front leg from 5-10 yards. I'd be curious how much lung a decent pellet can penetrate.