May I suggest that you try no 10 caps as they are just a tad slacker than no 11 caps. I use cci no 10 caps in both my B/P rifles both are kit guns by Ardessa and are the full size kentucky Rifle and the shorter Plainsman both in .50 calibre
I actually tried both 10's and 11's. I tried the 10's because I had some 10's in my cap loader to start with. The 11's worked better. But I feel like it's a nipple problem.