My suggestion would be to replace "Ruger rifle" with something more familiar to readers. As in "Winchester" and leave it at that. If a cartridge is absolutely needed for your story line, then go with another familiar and well known cartridge is the trusty old side kick .30/30. As was pointed out earlier, your lack of knowledge about firearms will shine brightly to anyone with reasonable knowledge of them. Your question points out your lack of knowledge being as if you can kill a whitetail with it, you can kill pretty much anything else both bigger and smaller. Shot placement is more important than any caliber/cartridge size.