My Coilgun Stage Driver Keeps Burning Out Mosfets. Any idea why?
Only guesses are possible without having your circuit. 

Guess1: When the mosfets are turned off the coil current bulldozes its way through D1 and D2 as you have planned. That leads to an accelerated mosfet Vds jump. Too high d(Vds)/dt can trigger parasitic parts inside the mosfets and that current route maybe doesn't stand what's available. Of course, I cannot prove that's the case. The phenomena is described in this paper: https://www.mouser.com/pdfdocs/Impacts_of_dv-dt_Rate.pdf You very likely could use the same ideas how thyristors are protected against too high d(Uak)/dt.

Guess2: The parts this cheap can be fake and do not fulfill the specs as you said in the question. But you hang at the edge of the genuine parts, too. See the safe operating area curve:



Your coil can have a substantial capacitance. Charging it too fast needs a  current which can be too high because Id and Vds are both high at the same time.