Most guys shooting smoothbores will get 5" or larger, at 50yards rest shooting and be lucky to hit consistently
on a 10" or 12" square at 100.
I've heard of groups better than 3" (benched) at 50yards with a smoothbore, but only saw that once
& that was with the 10 bore 1742 era Brown Bess Taylor built - & it was 5 shots offhand by me - a total
fluke, btw. Anything can happen once in a while.
The ball we were using was a .735" with .030" heavy ticking-like patch.
We have shot 3" groups at 50yards with a 20 bore smoothbore though, & I would say that was quite excellent. Note,
now and then you get a flyer or two in a 5-shot group - go figure. IT happens.
The US military, for their musketry, used turkey targets at 100 yards - yeah - go figure. With the issue paper ctg. in their .69's,
the average for a 5 shots, was one hit. By increasing the ball size in the paper ctg. to .65" from .64", they doubled their average #
of hits to 2 out of 5 shots - standing shooting.
A rear sight, as well as a blade for a front sight, should improve accuracy somewhat. Smooth rifles, ie: smoothbores with both front and
rear sights, were ordered just so, from stores in the West, from the Eastern gun makers.
I think it is best to think of a smoothbore, whether smooth rifle or with only the front sight, to be 50 to perhaps 70 yard maximum guns.