To ensure you don't mark the screw slots everyone turning a screw on a gun should have this:
(pasted from site)
MAGNA-TIP SUPER SETSTM
Mfr: BROWNELLS
Screwdrivers To Precisely Fit Virtually Any Gun Screw Slot
44 Custom Bits
9 Shank Sizes
You can now virtually fit any screw by simply matching that slot size and width to a precision, hollow-ground screwdriver blade from the Brownells Super-Set. This is the unique advantage of the MAGNA-TIP System's fast interchangeability; inexpensive replacement of a bit; and true, power-flowing, hollow-ground blades. The complete set of blades fits 99% of all gun screw slots.
With the Super-Set, you can pick the blade that exactly fits the screw you are working on. The Super-Set has 9 different blade widths from .120" to .360". Each 1½" long bit is marked with its blade width, and its relative thickness number (3 for thinnest, 4 for next thicker, etc.) and are arranged in a super, new, heavy-duty Bench/Range Case with a lift out bench Tray that displays the blades for you in a graduated "spread" from narrowest/thinnest through widest/thickest.
SPECS: Bits: Slotted- 1½" (3.8cm) long. Allen (Hex) Bits - 1" (2.5cm) or 1¼" (3.17cm) long. Phillips Bits - 1" (2.5cm) long. Handle: 1" (2.5cm) diameter, 8" (20.3cm) long.