From the reading I've done on firearms for the British and German colonial trade I believe you're on the right track, expecially with it being of British manufacture. I don't believe you will find the rifliing has been reamed or polished out. British Cape guns often had the smooth bore barrel and the rifle barrel regulated ot the same sight, for exactly the reason you suggest, one had a rifle for longer shots and the smoothbore loaded with ball or conical for in close, say from about 60 yards in, for critters with a less than amenable attitude. And, as has been stated, when not in the bush where unfriendly beasts make their abode you have a fine double gun for birds and small game.
A smoothbore with a round ball or conical can be very accurate up to about that range, 60 yards, and with a rear sight, nearly as accurate as a rifle with open sights. Best way to know for sure is to work up ammo and shoot it! (with black powder of course).
The 2 1/2 in. chamber was the standard for a looong time. Both shot loads, round balls and conicals were loaded in the 2 1/2 in. hull. Buffalo Arms carries brass hulls, the most likely fodder, for .410 through 12 ga., and I thought they used to stock 10 bore also. They're $24.28 for a box of 25 and will last a life time if shooting black. Here's the 28 ga. link.
http://www.buffaloarms.com/Shotgun_Reloading_Brass%20_it-161410.aspx?CAT=3832I think you ought to but the old gal back to work!
