Lyman has some 20 bore loads listed in #2 book that use 440gr., .54 cal. bullets in 20 bore shotcups. Some of the Black Powder guys with 12 bore doubles for waterfowl used to drop 1 or 2 20 bore balls into a 12 bore shot cup for moose while duck hunting - left barrel was kept loaded for moose, right barrel for ducks. Using a shotgun on moose or elk isn't legal here, but it has been known to happen. A double ball gun on the other hand, specifically for shooting ball would probably pass muster in a court of law. The rear sight would help considerably in the argument.