I bought a AR kit from a vendor that supplied a NEW US made barrel. It was in the white and not drilled for the front sight....I basically tapped the pin holes on the front sight base. Forget the size at the moment. once I did that and was happy with the position of the front sight I dimpled the barrel through one of the holes.
I used a tap I had on hand and the set screws where cheap. A lot easier than trying to drill the barrel and useing pins. you could also drill and tap up through the bayo lug tabs and use set screws there?
just looked, I used a 8 32 tap and I think the set screws where 3/16 or maybe 1/4 long. If you have to buy taps and drill bits might be cheaper to just buy a clamp on A2 sight base one from armalite was 45$ last I checked ? It worked well for me and it never shifted. I blue loctite the screws. I have since sold that AR and have never heard back from the buyer with any issues.
http://www.gswagner.com/arsight/arsightmods2.html here is where I got my info