Align the Pearce extension with the front of the Nano mag base plate as shown. Align both sides. A piece of tape on each side helps hold the parts together while you use a #25 drill bit through the Nano base plate to put a deep mark on the Pearce extension. Remove the tape. I used a drill press with a small block to level the Pearce extension while I drilled the hole with the #25 bit.
The depth of the Pearce extension must be reduced 0.125" around the rim. This will give an inside depth of approximately 0.10". This will match the old base plate so that the mag can be snapped into the Nano. I tried the oscillating sander method but I could tell it was going to take me until next week to remove that much tough plastic. I used the side of a 6" bench mounted grinder to remove the material a little at a time while continuously checking the depth with a digital caliper. Good luck.
