I belive DP hit the nail dead on. For me, under the circumstances of range, and game, I'd go with a 16 bore & smooth with sights - but only if I was going to do a LOT of shot/bird shooting with it. If I wasn't going to shoot a lot of shot, I'd go with a 16 bore rifle, and buy a cheap single fowler in 20 bore or 12 bore from/through Track for under $1,000.
I used my .14 bore rifle and a .58 rifle, with shot, for shooting grouse close in- ie: 20 to 25 yards max. It worked and I killed a number of grouse using the rifles that way. No, it didn't fill the rifling with lead. - I don't know why. I think perhaps any lead it might have picked up, was not ironed on very hard, and the first ball/patch combination I loaded (I like them tight), simply removed any lead on the way donw, or out.