It was common for many gunmakers of the time to sell smoothbores, often imported shotguns, and often stamped them with their names or got their names engraved on locks, etc. If the Hawken brothers made a typical plains gun in smoothbore, I'd be surprised. There's no advantage to a 9-11 pound, 24 or 28 gauge shotgun. It seems more likely such a gun, when we find it, was reamed out at a later date when the big game was all gone and somebody wanted to use it to shoot rabbits. The Hawkens were built they way they were for long deliberate shots over a rest or crossed sticks or deliberate offhand shooting- they are muzzle heavy at best.