Machine screws have three measurements: diameter (or gauge), threads per inch (tpi) and length. Gauge numbers run from #0 (small) to #10 (diameter of the cross-bolt in Weaver Tip-Off rings). You’ll sometimes find #12 and #14 screws, the latter about .25-inch in diameter. In “Unified” measure, diameter in inches = .060 + (.013 x the gauge #). So, a #4 screw is .060 + .052 = .112 outside diameter. 

Though 6-48 screws have long been standard for attaching sights and scope bases, stouter 8-40s are upstaging them. Kimber and other manufacturers offer production-line rifles with 8-40 holes. Top-tier makers of scope mounts, such as Talley, furnish bases for and with 8-40 screws. 

Another change for the better is the Torx screw head. Recessed like Allen heads, Torx heads have splines instead of flats, which are more easily stripped. Torx wrenches, splined to match, can apply levels of pressure that would ruin Allen or distort slotted heads. Besides L-shaped Torx wrenches, you can now get hard Torx bits for replaceable-bit drivers.