If you don't have a manual, strongly suggest that you contact Ruger to obtain one. According to the manual , it states that you can use either #10 or #11 or it's equivalent, whatever that means ??? It does warn that the cap should not fit so tight, as to have to force it on. So, I guess you would have to make that determination on yours. I use #11 on mine and per the set of replacement factory nipples, #11 is better.I use #10 percussion caps on mine.
Perhaps correct for your application but not true for others or most.#10 should be the correct size.