Before modifying your cylinder, I'd try the suggestion of using a larger size ball. You should be getting some "shear" - i.e. a small circle of lead when seating the ball in the chamber. And Doc points out a very good safety issue in regards to the balls moving forward and not being steed tightly against the powder charge.
You might try one of the suppliers and see if you can get a bag of 50 balls in a larger size to try.
What are you using for a charge? Have you tried cutting that back a little to see if you are still having ball movement?
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