What size hole, how many threads/inch for the buffer tube?

1-3/16 16tpi.
Drill/ream to 1-1/8" prior to tapping it.

1-3/16 16tpi.
Drill/ream to 1-1/8" prior to tapping it.


...goes to sit down with a grainger catalog...

Thanks Scott

Skip Grainger on this one, hit Travers or MSC.
Travers page 141 (they have taper, plug, or bottoming, your choice, ~$62 each) or MSC at $90-some on up.
http://www.travers.com
http://www.mscindustrial.com

There's a lot of cool machining info and suggestions on http://www.cncguns.com

I also found that I was hurting for vertical space, so I couldn't use a full length drill bit or reamer--I found a 3-flute aircraft countersink at 1-1/8" diameter instead that did a very nice clean job of drilling that hole, and it's a very short tool.

http://www.midwayusa.com/eproductpage.exe/showproduct?saleitemid=799763