Oops, spoke too soon. Put one of my other mags into the gun after I got if filed down for the first and the second mag became stuck. Had to file it down some more with the second mag in place (but after bullets safely removed, of course) but fortunately since I had filed most of the overhang down with the first, only took about 20 minutes of continued filing to get the second one out. Then I finished it some more to smooth it out. Now, both mags seat firmly and release properly.