The maximum internal temperature for a MOSFET diode is around 175 deg C. If they're saying it's 503 deg C then it should be completely crispy, probably along with the rest of your gun. MOSFETs have a negative temperature co-efficient so as the temperature goes up, the current throughput drops. It's what makes them immune to thermal runaway (a standard transistor has a positive temperature co-efficient so they pass more current when they get hot, which makes them hotter, which passes more current etc etc until it explodes) so if your MOSFET actually DID hit 500+ deg C then it wouldn't be passing over 100 A.
Either the Jefftron is fucked or the firmware is bollocksed. Personally I'd say it's the firmware update.