Broach? What's this broaching thing?
But seriously, I handled the mag well by getting a small handful of NEW, SHARP 1/8th inch drill bits, using them to drill the corners of the well, at 8 points, and never used one drill bit for more than two holes because they'll start to wander on the third hole.
Then, I used a largish drill bit (half inch or larger) to hog out a lot of the material between
the corner holes, and then I milled out from
corner hole to corner hole, taking maybe an eighth of an inch in depth per pass. This was really fun because the end mills weren't quite long enough, so I was playing the risk game by chucking up the end mills in the collet with very little gripping surface engaged. I also flipped the receiver over part way through as well to make things go a little easier.
I worked up close to the sides slowly and carefully, and finished out the corners with a few files, and I more or less filed the interior of the mag well so that any good condition USGI mag dropped freely.
Yes, it IS a lot of work to MILL out the mag well, but it can be done and the best advice I can give to you on doing it is to use high quality, sharp 1/8th inch drills for the eight corners, and don't use any one of these drills for more than two of these holes or it'll start to wander on the third hole. Really. And if you can get extra length single ended end mills, get them. You will appreciate having enough cutter length to cut the well in one setup.
I had a much tougher time getting the buffer tube hole right, but after one reweld and rework, it came out perfect, and I'd used a boring bar as a fly cutter (with a very carefully ground HSS tooth) to cut the 1.125" bore, too!
I'd post pics, but our AR15.com image galleries are so screwed up that I haven't been able to EVER upload a single image successfully since it was changed recently. If anyone can tell me about that, it would be appreciated.
CJ