Good shooting - the real test is 50 to 60 yards. With well balanced loads, smoothbore will stack them at 25 yards, just as will smoothbored long-guns, like the fusils and single barreled fowlers both with round balls and black powder. Something happens past 25 yards, but tends are good.

Most well bored guns, modern, that is, will hold onto 3 1/2" to 5" at 50 and that's good accuracy. I've heard of more 12's printing closer to 3 1/2" than not. I think that's one heck of a good bullet - cast hard and possibly quenched out of the mould (careful) or hardened with the hot oven and water quench method.