apples and oranges.

what you are comparing is in english terms a 20 bore shot and ball gun against a full bore rifle(577)
allthough it is fully rifled, the load is still a anemic shotgun round and not the 20 bore rifle rounds of the old days. difference in loading would be 1 drams(shotgun) to 4 drams(rifle) of black powder.

RBL Professionals make what looks like a nice gun, but it is just a rifled shotgun(i love and own those btw.) dont take it after anything more serious than pigs, even with dixie slugs(whose claims i highly doubt) remember that the gun is regulated for a specific load and changing any components may make it shoot way off.

IF RBL wants to play ball, get them to regulate it with a 4 drams BP behind a hard round ball and then you are into another playing field. then the gun should way about 9 to 10 pounds as well for you to shoot it with out getting tossed all over the place.

there is no easy or cheap way about getting a good bore gun that i know of, if there were i would tell everybody that i know.

best regards

peter