I have a 1975 manufactured ROA and use #10 caps on the stock Ruger nipples. The #10 cap fits snugly. I have used #11 caps in the past and pinched the cap to hold it on the nipple. Even then, I did not feel the cap was really secure to the nipple. Perhaps because I have a very early year production ROA. A trick I read online to retain a cap on a nipple is to buy some 'cap keepers', which is essentially a cutoff small piece of polyethylene tubing that fits over the cap on the nipple. Out of curiosity, I bought some on ebay and tried them out on my ROA with some #11 caps I normally use on single shot pistols. I found that the keeper does some tightening of the #11 cap on the nipple as it is a little hard removing both cap and keeper from a nipple. If you intend to try a cap keeper, be sure to force the keeper down as far to the nipple base for sufficient exposure the top of the cap for the revolver hammer to strike. The cap keepers appeared to work o.k. as the #11 cap didn't come off a nipple after several cylinder loads. There is a hassle with the cap keeper removing the spent cap and using it again. I don't know how many times that the cap keeper can be used repeatedly before it's useless. It's Interesting that cap keepers offered by several vendors on ebay say it is to prevent revolver nipple chain fires and powder moisture prevention which I have never read any tests of proof. There are a couple YouTube vidios of how to make your own cap keepers. I've read a few pros and cons about cap keepers, mostly cons from BP revolver purists who cringe at the idea of adding an abortion to their revolver, so the jury is still out as to the cap keeper. Except for the experiment that I did with the cap keeper and #11 caps on my ROA, I'll probably not use #11 caps again on my ROA with the small supply I have on hand of #11 caps for my single shot pistols given the dwindling cap supplies available at gun shops and online.