Ok. I got these all sorted out. I made 3 of them so far. The last one came out better than the first two. I ended up using a 3/8 endmill instead of a boring bar to finish out the holes and was much happier with the surface finish. I’m going to remake the first 2 so they all have the same barrel tennon size.
I started with.400 and it’s fine but the material is quite thin between the counterbore holes for the barrel mounting holes. I tried.385 on the second and the material between the holes was more stable but I got a little bit of chatter when boring the holes. Both of them displaced a lot of metal at the surface when drilling the holes requiring more deburring than I expected. I drilled pilot holes and finished with endmills on last one and it came out much better.
In retrospect, I should of made these first so the holes would be fully supported going into the aluminum fixture holding the part. I made the bolt carriers first so the holes are .420 and it caused a burr I had to remove on the barrel plate in all the barrel holes. Not a big deal but one more step that could of been avoided.
