I had a lead ball behind an iron tipped rr, 50" barrel, .54,40g 3f, maybe 10th shot from a new rifle. I seldom use the rifle rr at the range except for this time. The lead got around the tip and locked it up. The forward 16"s of rod snapped off and hit target. Still makes me cringe.
With a proper patch and ball fit in the barrel, snug but not overly so, it doesn't matter how long the barrel is. Rate of twist being the same the air born ball will obturate the same.
Sorry Brit, off topic.
1994 the tenth printing of the Lyman Black Powder Handbook.
" Nearly equal velocities are obtained through the use of a notably lighter charge of 3f powder.
We then decided to use 3f in all but .58 and .75 in our testing. 2f in .58 , .75, 12 and 20 gauge."