As I continue to mess around with HHWs I've discovered another error caused by them. If you max out the fire rate on a RAC included in a handheld weapon it will generate a runtime exception that will cause the game to become unable to proceed and kick any connecting players.
I've tested it with RAC2s and RAC5s.
I've opened an issue: https://github.com/MegaMek/megamek/issues/8048