Boy, was I wrong about the five minutes!!!
After spending two+ hours today, I have the mag body slotted! A long tedious chore, with one broken 1/8" 4-flute end mill.
It just wouldn't take the feed.
(BTW: Mec-Gar mags are not hardened). I ended up doing it all by hand, very slowly.
I made a relief cut in my tube insert, then clamped the mag tube + insert in my 'sandwich' and indicated off the tube front for my slot. I used a #31 drill bit to zero my 1/8" end mill and sank it into the hole. Then it was one cut up the tube to open the slot up. After that, it was just climb and conventional milling each side to open it up a bit, as I'm planning on making my button pin 1/8".
The first end mill broke off when I was ~5/8" into the power feed, as slow as my Chinese feed could be set. It left a jagged edge that cleaned up with subsequent cuts. A bit wider slot than I wanted, but them's the breaks [bad pun].
Next up is the follower.
