Types of rail screws, best options.

There seem to be 2 sizes at least. What are they? Thanks!

6-40 socket head cap screws seem pretty common.

Yep 6-40 are std for every rail I've  deal with. If you encounter any issues with those, 8-40 is the common upgrade size

Quite a bit depends on the number of threads engaged. A screw will have full strength when 3 threads or more are in use. Another issue with screws has to do with the length of the base being attached to steel. Aluminum has about twice the expansion rate as steel. I make my bases so if it's long I use flat head screws on one end and either button head or caps screws on the other in slightly bigger holes. I use 8-32, if possible I up-grade to 10-32. I avoid 6-32 and use 6-40 if I must use smaller size. 6-32 is a size that likes to break, I have broken more 6-32 than any other size. I do have a tapping head that will accept only one size tap in the holder that is preset for the acceptable torque limit so if using it I will not break a tap! Spiral flute bottoming taps are best for blind holes especially short length like a barrel so threads go as deep as possible. I like OSG brand taps for spiral flute bottoming taps. I only use High Speed Steel taps though I do have a few carbide and cobalt taps. Carbon Steel taps will become dull sooner, break quicker but because they break easier they are easier to remove if broken.

Rail screws are primarily loaded in shear and the biggest load is from the inertial effects of slide operation. The “grade” of these screws is revealed by the recommended torque values in the installation instructions.  For arguments sake their “grade” strength-wise is 5.  That means at the recommended torque the clamp load of the #6 screw is 2/3 (600 vs 900) pounds and the shear strength is about 2/3 of the strength of the #8.  

So... if I was going to run a heavy dot on a steel rail, I’d likely use #8 screws.  Or at least #8 s near the attach points of the dot.  Another reason to use smaller lighter dots.

I went straight to 8-40's on my 45, but I made the rail myself so could make the holes any size I wanted.

I like the Torx screws from Brownells. Much nicer head size than a standard socket screw. The other option is a low head, but McMaster only has them in 32 TPI. And the low head screws have smaller sockets in them, which are easier to damage than a Torx.

As Froneck notes, more threads engaged with a finer pitch. Good idea for any hole, likely shallow, put in a barrel. And a 1911 slide isn't terribly thick either. BTW, if you drill the rear holes .3" deep you won't break into the firing pin channel. And you probably won't need a bottoming tap either. Although I do grind the points off of plug taps for some free depth.