I currently--and probably forever--own a .50 smoothrifle, Peter Berry-ish, using a 46" Longhammock barrel. Great with #6 or smaller shot on little furries or roosting grouse. Shot a Columbia blacktail doe some years ago at about 30 yards. Did the job.
A friend built a .40 flint smoothrifle years ago for hunting in a shotgun-only area. Since .40 was the smallest muzzleloader allowed, he figured he had both requirements met. He never had a game cop stop him, so we'll never know. He did take Columbia blacktail with it nearly every year.