something to consider
You might try bedding a pillar into the wood. It sounds like maybe the wood is compressing on you due to the torque of the screw and the vibration of shooting the gun is what is causing the screw to come loose on you. If you didn't want to do the pillar yourself, a smith could do it for a smaller fee than you might think. You could try glassing in a washer the same size of the screw head flush with the wood so the screw has something besides the wood to bottom against. Good luck with it and let us know what you come up with.
