Spent a few days mentally trying to picture how I wanted it to look. Clean and simple won. Settled on vertical slots to play off the picatinny rail. I'll offset the slots and cut them relatively shallow so it doesn't look like a quad rail when finished.
Used a 0.250" four-flute center cutting endmill. Each slot was cut 0.394" on center from slot number one. I wasn't trying to duplicate a picatinny rail, it's just an idea to break up the plain sides of the breech and add some visual interest.
Deburred and sanded to 220 grit.
The tan color inside the bore of the hole is motor oil. It's swimming in it at the moment.
Sanded out the machining marks from the left side.
More layout to match the opposite side. Used a feeler gauge (and some arithmetic) from the top edge of the vise jaw to set the milling cutter at the same depth as the other side. Cutting 0.050" from the top of the vise--about the thickness of 12 human hairs. Imagine that it's probably considered bad form in machining circles to mar the vise. Luckily, I'm not a machinist.
Cut these at a high speed and in a single pass.
A very quick deburring and here it is.
Top view.
Not a great detail shot, but I added a lone slot behind the bolt cut to use up some of the blank real estate. It was intentionally cut to not match the other side--as there wasn't a way to make that happen with the given space. However, the front edge of the last slot does center on the opposite slot. It sort of shows up in the top view two pics above.
The breech needs more polishing then bluing and the shroud still needs to be cut to final length and maybe ported? then finished. Might need the spin index after all. We'll see what else shakes out.
More soon.