Drilling a hole in a pane of glass or a mirror is simple. The key is to use a carbide bit made especially for glass and tile (see photos). You’ll find these bits at most home centers alongside other drill bits or ceramic tile tools.
Because glass is extremely smooth and hard, the bit will want...
What magazine are you using ?
How tight is it to insert a mag , I have had several stocks that the mag well area had to be enlarged.
Also the original push up mag release on metal housing trigger groups is not compatible with newer mags.
Is this a SBR with a KAK blade ?
Rem 870 will feed 2" mini shells, not the 1 3/4' currently made. The Mossberg with the adapter seems to be fairly reliable.
https://www.sportsmansguide.com/product/index/nsi-minibuck-12-gauge-2-1-4-00-buckshot-6-pellets-50-rounds?a=669205
https://www.5dtactical.com/ar-15-80-lower-jig-p/5d-jig15.htm
this is currently the most fool proof method of making a 80 percent
there are about 3 jigs that work well they are all in the 150.00 range and the specified router for all of them is about 150 to 200