I have a real love hate relationship with the .62 caliber smoothbore. I have the same GMB that Roundball has in percussion. I have tried countless different loads, manner of loads, wads and no wads, just about any way you could think of shooting it. The best day I had on the range was the day I shot that group there. That was at 43 steps that I later shot/measured with a range finder to find it was 39 yards. After that distance I do not trust the rifle for accuracy. I did try shooting it once at 75 yards and actually missed the paper the target was on. Where it hit, I have no idea. The second shot I managed to get on paper in a corner. And also managed to fling one ball downrange and actually make a good hit. But that was because it was spraying all over the place.
For a shotgun, I load 80 grains of Goex and then push a Winchester AA plastic wad and then load 100 grains of #5 shot. It will really shoot with that and would be more then enough for 25-30yard shots on about anything I wanted to hunt. I was rolling a paint can around the yard one day and was surprised it would penetrate that paint can. I figured a bird had no chance at all.
The top picture was some penetration tests I was shooting. I shot all the way through that chunk of Balsam length wise. Also I recovered a ball that had hit the steel bullet trap. The splatter effect was really impressive. I think that would have been an excellent short range deer load. I just never would trust the rifle past 40 yards.
Like Roundball said, the versatility of the rifle was a reason it was so popular with early settlers. They could big game/small game hunt, bird hunt, basically hunt anything they could get close to, all with the same gun. Early settlers were not gun rich from what I read. They were lucky to own anything. Actually a lot of old military muskets were popular with early settlers because of their cost. Rifles were expensive actually, probably since most were individually made.