You can buy a small endoscope off EBay for less than $15. If thin enough, you may be able to see the slot and when your screwdriver slips in. Better than breaking the stock.

I'd tape it right on the screwdriver, a small 2mm scope might go into the hole. If you use weak tape, you'd be able to pull it back before turning.

Handy to use for checking for bore pitting as well. But, you have to have one with a little 90° mirror attachment.

Regards
Ken

Last edited by Ken61; 02/19/17 07:22 PM.
I prefer wood to plastic, leather to nylon, waxed cotton to Gore-Tex, and split bamboo to graphite.