I had the same problem. As I pulled the trigger, my trigger finger would come in contact with my thumb producing a flinch.

Try this. Don't bend your thumb. Extend your thumb straight forward. Now you won't contact your thumb with your trigger finger. But you have to learn where to place your thumb so it doesn't get torn up by the cylinder release. Extend your thumb below the cylinder release. Sounds more complicated than it is. Practice a bit and it becomes second nature.

__________________
Accuracy supercedes Speed