I've only found one source where the drill bit mentioned for spacing the rails is specified.  It said, "C" size.  I know what size it is, but can't find it at the local places.  Is there a suitable commercial size people are using?

caspian

I forgot exactly what size but I just kept going through my drill bits until i found the right one that fitted without the rail binding against the receiver.

The funny thing is that, in my opinion, the rails were the easiest thing about this WHOLE project. Hardest part(s) for me: The trunion holes and that damn trigger guard. What a suckfest. My magazine doesn't wanna go in because the trigger guard flexed forward.

15/64

There is no "absolute" size drill to use to line your rails up.  Here's what you do:  Put your rail in the receiver and put a drill in there that lines it up.  The drill is there to keep you parallel to the top rail and to maintain your alignment with the trunnion.  The drill size can vary widely depending on the kit.  Just use whichever drill gives you the correct alignment.