You can use a regular spark plug feeler gauge to set you gap. There are a few tricks you can try to wring the absolute best accuracy from your revolver. The barrel cylinder gap is one of them. I have the gap set on my .22 and .41 so the cylinder can turn with out hanging once a bunch of crud builds up on the cylinder face. Be aware of the change of impact when you change the gap. With my .41, the point of impact doesn't seem to change as much as my .22 when I diddle with the gap. Be aware of the change of impact.