My grip screw basically sheared on both sides of the slot, and yes I used the correct driver and technique. So I just drilled off the rest of the head and replaced it with a proper Allan head screw after refinishing the wood. It took an unbelievable amount of torque to remove the T nut from the grip screw once I had it out of the receiver, no way a flat head driver would have moved mine.
Just had to tap the forend wood down once the screw an rod were removed.
Sounds like some have a lot more thread lock than others. The screws on mine were tight but broke free without too much force.