maybe a .260 or 7mm-08... I know a few people that use a .243 on whitetail deer, i just think it's too small. We hunt in brush and the woods and to be honest, getting the perfect shot is not usually in the cards and sometimes they will hold back so they don't take the chance of wounding the animal. Will a .243 kill deer, sure, many hunt deer with it. But like mentioned above, make sure your shot placement is right on.