Minor gunsmithing question

[xxclaro]

Okay, so a few days ago, with the advice of some of the members here, picked up a 8-40 tap to retap the front scope mount holes on my .307 BB. I need a drill bit to match the tap, but the catalogue I ordered from didn't specify a size. Does someone know which is the proper bit to use? Also, those front holes are pretty shallow, terminating at the barrel shank. The threads of the barrel are visible, and have been flattened by past scope mount screws. Do I need to worry about the drill bit touching hte barrel shank? I'm sure I wouldn't want to drill deeply into the barrel, but is a minor dent going to be an issue? Thanks for all your help.

[Greg807]

I think your looking for a #28 bit, that being said, I hate the wait and the cost of a gunsmith just like the next guy but there worth every penny most of the time.

[Malamute]

You probably have a regular (starter) tap. You'll need to start the new threads un til the tap bottoms out, then grind the starter tip down some, tap some more, then grind the tap down some more to bottom the threads.

If you have a hardwarwe or machine supply store around, go look at the middle and bottoming taps to get the idea.

You may be able to find pictures on;ine to compare the different taps.

On an open hole, it doesnt matter, but on a blind hole, you'll need to get a bottoming tap or grind yours down after getting the threads started.

[Pete44ru]

The original holes were D/T'd before the barrel was installed - You don't want to drill any deeper into the barrel than the depth of the existing barrel thread roots. Remember - you're at the catridge head area of the chamber, where pressure is pretty high, relatively speaking.

[marlinman93]

Save yourself some grief and buy the bottom tap. Sooner or later you'll need both taper and bottom taps in that size anyway.
A #28 bit will work, but a #30 is what they sold me to go with mine when I bought that size tap. It's a bit smaller hole, so tighter threads.