The narrow slot that rides into the chamber was another matter. This is about 3/32 wide and a good .2" deep. Just getting the piece set up was interesting enough; I ended up with a stack of parallels to raise the bolt up sufficiently for quill clearance, with another one slipped in against the previously milled flat in order to achieve the 90-degree alignment I needed for the last slot.
I purchased a 1/16th end mill with a tapered shank (the only thing they had close to the right size) for this, and that was a bit scary. Put the RPMs up to 3800, used the paper trick to get the height. According to my very rough calculations, my cut depth should be around five thous per pass, and my feed rate "really slow."
There was no chatter or smoking coolant, so after a dozen passes I increased to ten thou per pass and a comfortable feed rate. Doing this by hand, and being careful to advance at a steady rate and not slam my tiny fragile end mill.
Widening the slot I was down to less than a quarter of the mill diameter so I could afford to stick a lot more length in there. That went quickly.
And the bolt fit. A little looser than I'd like, but I'm ready for welding now.
I also had a last grenade to run off -- slightly smaller dimensions overall so it would fit properly in an Airsoft shotgun. I thought I had a chunk of 7075 left but the small chips and lack of sound told me this was more of the 2011. (The main differences I noticed working 6061 is longer, better-formed chips, and a tendency to "sing" -- vibration noise.)
Unfortunately, the cross-feed gear was not engaging correctly -- it was as if it had a backlash of half an inch or so -- and I couldn't make accurate cuts (also, the runout was on the order of 60 thous). So I contacted staff, they took a look and said they'd probably have to order parts. And I waited for a user to clear the other lathe and hopped on that one when he was done.
And mucked up several of my cuts. I'd gotten too used to the mill or something and was consistently over-shooting. And I hope I cut it small enough so it will work.
The funny part is, though -- the guy on the other lathe? He was working from my Instructable!