As far as cutting the radius,
I had this plan.I have a Swenson barrel to slide centering jig.I was going to depth mic from the flat side of the slide to the barrel,with the barrel centered.
Then,by mic'ing the bbl OD,I have a radius and a location to set up in a Bridgeport.Or,I could just indicate center on the slide.
I do understand I don't want to jam the bbl to one side to take all slop out,just nudge it to center.
I realize a ball mill large enough to cut the radius will not be able to get too close to the breech face.
Going with someting like a 3/16 carbide ball mill,I know how to generate a chart of numbers to crank handles point to point to match the bbl radius with a small ball mill.
That's how I used to cut mold cavities old school,before we had CNC in the shop.Then I just stone off the high spots.
So,I could make the Swenson insert,and the little tack of weld on the outside of the slide wouldn't make much heat.
AAAHHRGH!!!I suppose I could buy a bbl,and indicate the slide stop hole and spot the underlugs with an undersize end mill,then pick that up.
In the mold biz,we had occasion to use the services of microwelders.Those guys are amazing.