I have a small Grizzly milling machine that would make that job you're doing a piece of cake. I have a Kurt 6 inch vice that just fits the table in my cellar shop. Grizzly has fairly inexpensive Chinese coated endmills. I bought 2 sets. The mill is manual. I setup dial indicators for the fine moves.

When I did my tool and die apprenticeship, the 1st year was all hand bench work like using files to clean up jobs like your milled out recess, then to Swiss files and mirror finishes---before even turning on any of the machines.

Got to "play" with a couple of shotguns today...Firing off ~23 shells kinda hurts, but well worth it!

imgur.com/YgmJiDe

^One of the targets I hit...This one ate most of the fire. The other one was about 50-60 yards out, only aimed at it once.

imgur.com/ZlIgLBW

Just a shell sitting on the desk...Along with part of my phone charger!

Edit: Trying to fix the links...

That milling jig looks familiar. I modded a piece of aluminum high-tech alloy blank similar to your photos for a buddy; I used a milling endmill. Just clearance for the trigger assembly. His dad left him an 80 acre farm, mostly woods, upstate central, NY State out in the middle of nowhere to have fun with. Not going into specifics due to possible prying eyes. Wonder how many cops in plain clothes, local, State and Federal are monitoring a typical hobbyist gun show?

I prefer a 12 gauge, 9 round pump shotgun, shorty barrel for home invasion protection. Just pumping that thing to load the chamber, most guys would take a dump in their pants and quickly leave! I would hate to have to kill somebody no matter what the reason.