Ruger Bob from ME (I bought an antique gun at Kittery Trading Post last night), I drag this out whenever a cap question comes up. Basically the higher the number, the bigger the cap. All #10 that I have bought are smaller than #11s but others will tell you that this might not be true. I use caps that have to be pinched to stay on the nipple - a smaller cap might fit until you fire the gun a few times and the nipples foul up and get "bigger." Anyways, read this:
From an old Colt Industries pamphlet:
"Percussion caps are now made in sizes from nine to thirteen. Ten and eleven are the best numbers for the small and medium-sized arms, and twelve for the larger sizes, although, as different-sized nipples are sometimes met in specimens of the same model, no hard and fast rule can be given. It is better to have caps slightly too large than too small, as large caps can be pinched together at the bottom enough so they will stay on the nipples, but small ones must be driven down on the nipple by the blow of the hammer, and this process frequently cushions the blow to the extent of producing a misfire."
