Probably about 50 or 60fps difference - rifle faster of course, with black powder or smokeless. Just a guess.

We tested a .62 ML rifle against a 20 bore ML smoothbore, using the same diameter balls and powder charges, however the rifle needed thicker patching, of course.

The rifle barrel, although 8" shorter, gave slightly higher speeds. Both were tight loads, needing a ball (short) starter to assist loading. At that time, we did not have a shorter barreled smoothbore to compare equally with the rifle. Now that I do, we having re-tested this comparison, smooth vs. rifled. Due to 'some' velocity loss with the shorter barreled smoothie, the difference will quite likely advantage the rifle even further.

Much likely depends on the powder used - military, vs. high grade sporting powder, I'd expect. Compared to the best BP's of the 1800's, American DeadShot in the States and C&H #6 Rifle Powder in England, our current GOEX is likely equivalent to mil-spec powder of the 1850's.