I just read a post about a chap who had seized screws on a scope setup. In the case of phillips, pozidrive, or even old fashioned slotted screws try the following before you completely wreck the head: Pop down to your local motor factor and buy a small tin of valve grinding paste. Dip the tip of the screwdriver in the paste and try again. The carborundum paste bites into the contact face of the screw and driver giving much greater friction, and therefor much less chance of the bit slipping out of the groove/slot. Also be sure to buy a new, and correct bit or screwdriver.
Oh I almost forgot,,,,,,,,DON'T GET ANY GRINDING PASTE ON THE WORKING PARTS OR BARREL OF YOUR RIFLE, AND IF YOU DO, BE SURE TO WASH IT WELL CLEAR BEFORE USE.