I shoot several 1858 Remington replicas (Uberti, EuroArms, & Pietta) and dearly love them all, but capping is a real problem.
NMLRA pistol matches don't allow capping until on the firing line. The Cash snail version doesn't work on my pistols as the jaws are too big to fit the nipple recess, but the older Cash straight line with two jaw springs works OK, although a bit technique sensitive. The newer straight line cappers use one jaw spring and extend the other side of the capper ... I found filling away some of the solid brass "other" jaw, it can be made to fit.
If you're just shooting for fun, try capping the cylinder out of the frame. Its easier to handle and re-installs quickly. But you must be very careful handling the capped cylinder as you now have a 5 or 6 shot short barreled pistol. DON"T DROP IT!
Capping on the firing line with a tin of caps in one hand, and the pistol in the other is another choice. For me, about 50% of the caps get fumbled off and onto the ground. You need a brain surgeon's dexterity for this trick.