cutting and rethreading mags isn't as easy as it may sound, but it isn't hard either.
the threaded portion on the top makes them impossible to chuck properly, so you need to thread them into the head, and chuck the head. with the head chucked, it's impossible to get the tube to spin 100% centered.
of course, i've got a small lathe, and if your lathe is large enough it should be possible for you to chuck directly onto the tube.
as far as the process, it's rather simple.
use a cutoff tool to score the light wehre you want to cut it, then use a hacksaw to completely remove the bottom end. (using the cutoff tool all the way cost me several mm's, because they tend to get ripped out of the chuck)
after cutting it down, bore it out a little bit, so you can get your tap into it. then tap it, then bore the end for where the oring can go.
a tap is the best way to do it. internal threading is a b!tch! the appropriate size is 5/8-28
i'm somewhat of an expert on the subject, having cut down over 50 minimags /ubbthreads/images/graemlins/icon15.gif