I know it's been discussed before, but what if the method of "boring" was changed.
I'm thinking a reamer of some type is fixed in a lathe, the mag body is slid over it and fixed in a clamp that allows it to be moved up and down some. The reamer is then supported on the tailstock end and then spun. The mag body could then be pushed down onto the reamer leaving a "bored" out spot along it's length. Loosen and then rotate the mag body 1/3 or 1/4, depending on if you want tri or quad "boring".
I don't know for sure tho, this would probably require a special clamping jig to secure the mag body. Then you'd have to source a long enough reamer...although I think a tooling company could make one...it's just a rod with grooves cut in it and the fingers then sharpened.
(Edit : fixed some grammar mistakes)