This is quite a common problem. Fortunately, the taper is quite steep so not a lot of force is required to separate the AR from the mag. If you been unscrewing and the internal mechanism hasn't released I would recommend the following..... Screw the nut back in, but not very tight, just finger tight should be OK. Drill and tap the centre of the nut to take a 5/16 bolt or 7mm, unscrew the nut until it is loose again then run a 5/16 or 7mm bolt into the threads you've cut, Hold the ATD nut firm and square and tighten the 5/16 or 7mm bolt which should put pressure on the armature shaft and extract the ATD.