Yes ,a 22mag is very capable of taking a whitetail.Ive seen it been done before.Im not encouraging it,as it is illegal in every state I know of to use a rimfire cartridge to hunt deer.But it is definately capable.The indians of the far north kill the BIG bears with it regularly.Thats right ,the big bears with a 22mag and 22lr.Dont ask we why ,but they do.They sneak up and pop them in the head at 3 feet and it goes through the brain.If you want to think about it in a technical point of view,you can kill any north american animal if your cartridge can penetrate 1 inch of bone.The one inch is the skull thickness,and most deer only have around a 1/4 inch thick skull.If you can get to the brain,Its dead.The only problem is getting really close to the animal.I know someone that was forced to shoot a medium sized black bear with a 17hmr.One shot through the eye and into the brain and he was dead.Ive heard some people say the 17hmr is only good for animals less than 5 lb.That bear didnt seem to know that.What really maters is shot placement.