800 FPS?!?!?! Man that's haulin' the mail!!! Sam and Gene are right,...I wouldn't bother with the shims either. Most of the time they aren't necessary and inhibit correct barrel lock as well as add to barrel droop. To me, if you take a straight edge, and drag it over the breach,...and you feel the seal,...that's good enough. The tissue test is a good way to tell if you have 'blow outs' around the seal, but a visual inspection will tell you as well. I call it the 'dough nut' ring around the port in the end of the action (for rifles that seal from the breech). If there is a nice clean ring around it, chances are you're good to go. If you see where air/exhaust is blowing by the port and seal like a smear around the 'rings' outer edge then a shim might be necessary. I rarely clean the breech area of my guns, so a minute amount of dust helps with the visual inspection. I do wipe off the seal face periodically though. Hope this helps, tjk