I’ve got a Ruger Old Army that I’m having trouble with. I load 35 grains of Pyrodex P under a plain old Hornady round ball and a dab of Crisco. Recoil causes the balls in the remaining chambers to move forward and lock-up the cylinder.
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.
Flint is right. You are getting "ball creep" and need to very slightly bevel the chamber mouths or scrape the edges to remove the lip so the balls are swaged rather than sheared when seated. Sometimes real hard seating will flatten the ball a bit to swell it into the chamber but better to open up the mouths a bit.
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