I also use 158 grain RNFP bullets by Scarlett in my 1866 and have never had problems with them. I load them with either 3.2 or 3.4 grain Trail Boss (yes, I still have some) to an overall length of 1.50. I've also used 125 grain truncated cone bullets by Scarlett and others in the same gun to an OAL of around 1.45" with the same Trail Boss load, again with no problems.