I have noticed if I go hotter on the load I get tumblers. I recon this is because the fast burning powder is stripping the dia and its not biting the rifling.
A cure for this would be to use a harder alloy as we would for hotter cartridges but this creates problems. First its too much strain on the loading lever so a bench press would be less damaging to the gun. Second if you have a mishap and have to draw the ball it wont be as easy to screw an extractor in.
Alternatively reduce the strain and use thicker wads. Or stick to light loads. Or go back to balls as John says.
“If a cricketer, for instance, suddenly decided to go into a school and batter a lot of people to death with a cricket bat, which he could do very easily, I mean, are you going to ban cricket bats?” :- Prince Philip said after Dunblane