The Pedersoli Mortimer-Whitworth and Gibbs rifles are high quality. The 1:21 twist M-W is meant for ultimate accuracy at 100 yds using their pretty blunt .45 500 gr. bullet of long-shank, multi-groove design and moderate powder charge of perhaps 80 gr. volume Swiss 1.5 F powder. If using ordinary BP like Goex, perhaps 90 gr. volume. Comes with the shorter tang sight shown in above pic. Remove that sight, and its base set in the stock, and you can install a tall tang sight such the modification gunsmith and ML shooter Lee Shaver makes to the available Pedersoli long range tang sight.
I shot a M-W model before the Gibbs came out, successfully, to 1000 yds. But I used a L.R. bullet of about 540 gr. and a heavier powder charge of 107 gr. of 1.5 F Swiss, to give adequate spin rate to the bullet for long range use.
The Gibbs is 1:18 twist and meant to shoot a bit longer .45 bullet of about 540 gr. having a longer nose with good ogive form, and meant for 600-1000 yd. shooting. Powder charge often about 90 gr. by vol. of 1.5 F Swiss powder. It comes with, or did, come with a highly adjustable long range tang sight. Best to have a platinum-lined nipple if using a heavier charge and heavier bullet like this, due to increased pressure and therefore faster wear-out of a nipple.
Both rifles previously had a slight choke at the muzzle to maximize accuracy, say about .001" constriction. This is finesse manufacturing, and probably the result of precision lapping after the rifling is made. These models probably still have that, but ask Pedersoli.
Aloha, Ka'imiloa
.