Sorry to be tardy in replying, been working late lately. If you let the grit side of the tape drag up and around the edge you could round it, I guess. I have always supported the tape with the same hand that I use to hold down on the buttplate, or in this case extension. Sounds awkward I know, but you can do it. Just let the tape ride over a finger as it enters the joint.
When I built muzzleloaders this was a revelation to me. It worked so well that my buttplate fitting went from fair to very nice immediately. Never had any problem with rounding the edge off. Remember, the first thing the tape cuts away is the high spots. It's no trouble, for me, to pull the tape straight out the side.
Stan
