I had the problem with a Stainless Old Army, and no size ball will cure it, as it is the throat of the chambers. Somehow Ruger manages to burr the opening enough to cut the ball diameter to less than the main chamber, so they are loose once seated deeper in. Take a triangular machinist's scraper, or even a dowel and 320 grit wet or dry sandpaper and clean the front corner of the chambers. I sent mine back to Ruger, and all they did was scrape the chamber, I can see the marks, and that, I could have done myself. Take a caliper and measure the chamber at the throat and further in, and you will see the problem. Good luck. Another hint, use Speer, not Hornady balls, which are not round.