Get a "push stick". A 3/8" dowel, rear end of a BIC pen, etc. to push the caps further on. They should not rub on the recoil shield at all. To me, that means they are not yet fully seated and are sticking out. You may have a strong hammer spring that is hitting hard enough to smash them down both seating them fully and setting them off.
With over 15 perCUSSIN' revolvers, I've been called the Imelda Marcos of cap & ball.
SASS#3302 (Life), SASS Regulator, NRA (Life), DGB#129
Wolverton Mtn. Peacekeepers (WA), former Orygun Cowboy (Ranger, Posse from Hell)