Never understood this one. A 308 doesn’t gain any power from any more than a 20” barrel, and I don’t see how a black powder weapon would.
It all depends on the powder burn rate. This article says that an optimum length in a bolt action .308 is 26-27 inches.
There were many shorter barrels on the old guns.
The longer barrel also gave the shooter a longer sight picture for more accuracy.
Danial Boone and Davy Crockett never shot squirrels out of trees at 200 yards, but could consistently hit a man size target at 100 yards, if it wasn't too windy, too still, too damp, too dry, too light, or too dark.
The early grades of black powder were very poor. They had different formulas, depending on the manufacturer. They were not granulated and protected by a covering of carbon. The sulfur, charcoal and saltpeter tended to settle out and the barrels had to be periodically rotated. Thompson Center ran tests with MODERN black powder and found the optimal length of a barrel was 28” beyond that, the extra length gave no mechanical advantage. But there were no chronographs available back then, so who knew?
BUT there were OTHER reasons for the long barrel
1) A long barrel tends to muffle the report. If you are hunting in the woods you don’t want to let every Indian in the area know where you are if you are white, and if you are an Indian (American Indians were big customers of long rifles and trade rifles and according to some authorities contributed to the evolution away from the Jaeger type guns by requesting modifications in the gun to suit the new world forests) you don’t want to let enemy tribesmen or hostile whites all over the area know where you are.
2) With open sites, a longer site radius makes the gun more accurate.
3) The artists - and they were artists as well as woodcarvers and metalsmiths - who made these guns followed the “Golden Mean” in their construction and a longer barrel made the gun more appealing.