They are hard to come by. I solved the challenge by getting a 0.001 shim from a mechanical supply store. Actually I got a 0.001 AND a 0.002 for a buck a piece. They are bigger and wider and probably better for this application. The .280 barrel I got off this site fit just fine! The good news is I got a nice buck with it. The bad news is I didn't get the chance to REALLY test it because he was only 50 yards away.