They might fit a little tight on your #11 nipple, but they should still work. I always used the hammer to fully seat the caps anyway. Quite a few failures to fire were caused by not fully seating the cap on the nipple, and the space absorbed the impact of the hammer. I even kept that procedure up after I changed over to Musket Caps... Traditionally, #10 caps were for pistols, and #11 caps were for rifles, but I never could see a big difference in the two.