RB's can be very accurate out of 1:48 twist barrel. The secret is to keep powder charges lower. The RB needs a slower Rev Per Second. Similar charges would be: 100gr-2f in a 1:66 = 70gr-2f in a 1:48. The velocity is slower in the 70gr charge, but, the Revolutions per Second, down range, would be the same, thus, the stabilization/accuracy would be the same. Sam Fadala explain's it quite well in a couple of his black powder loading manual's.
My friend Buzzard has nailed another one!
1:48 will shoot a Roundball quite well....however, if you try to push it too fast, ie to much of a powder charge, you will run into accuracy problems.
Several theories on why this is, but the most prevailing and most likely theory is "skipping"...skipping can be seen in the barrel as "galling" and patches will definitely show a skip, if you find that patch
However, it is hard to duplicate intentionally...if you are getting shotgun patterns with your rifle at 50 yds instead of actual groups, skipping should be a strong suspect for the reason.
Of course there are a dozen or more reasons for really bad groups, but one of the major causes is too much powder, and a patch that is far to thin, or way over thick.
My 1861 Colt 3-Band .58 cal with 1:76 progressive riffling was the hardest gun ever for me to get a grasp on...I was always using too much powder. It shot a round ball best in the early days, but I later got the Lyman 575213PH (Parker Hale) 560gr Minie Ball to do what it was supposed to do....
without blowing the skirt on the bullet......
With Roundball, I started with 60gr and went to 120 gr in 5 gr increments, using several 'different powders' and many different patch thickness / material, and two years later I discovered that what it all boiled down to was a very accurate load using 43gr FFFG Goex....1" groups at fifty yds, consistently and 2.5" groups at one hundred yds, also consistently.....a load I picked up on the NSSA web site.
It seemed all the rest of world knew this but me....the later on I discovered that not many people at all understand the RPM / RPS (Rev Per Min / Rev per sec) principal...ya gotta keep the ball or projectile within the grooves / rifling of the barrel, or disaster is sure to follow.
With all the "hot loads" I was getting galling and didn't even realize it until I got my first bore light....and it was bad.
Uncle Russ...