I'm having some trouble getting into field stripping my Mossberg 500 Tactical. The barrel comes off fine, but as soon as I try to get my pistol grip off I run into trouble. The tactical stock pulls right off but the pistol grip looks like it's being held by a circle-shaped screw of some sort. I was reading on some other forums and they said something about it being a hex-screw, that's a no-go. I've tried seven different hex-screwdrivers and no luck. Does anyone know what I can try to get the pistol grip off so I can access the internals?