If you look at an M-16 and a Mini-14 mag side by side, you'll notice that the Mini-14 mag is much narrower up front. You'd have to do MAJOR surgery on an M-16 mag to make it work in a Mini-14. (I have a Mini and some M-16 mags leftover from when I had an AR, so I looked into this.)
I concur with jthuang -- just get some PMI mags. That's what I did.