EDIT: Solution 0.63"

Hi all,

Just a quick question: I am having my PC Colt turned into a non-PC with a flash suppressor at a local gunsmith.  While he is very knowledgeable and is known for making great target rifles, he doesn't have a lot of AR experience.  While he could install the Phantom FH just like any other muzzle brake, I'd like the threading as close to spec as possible.  So....

What are the dimensions of the threaded section from the muzzle the shoulder?  I know it's 1/2 x 28 tpi, but I don't know how far to turn it down in the first place.  Could somebody tell me?  Maybe just grab that spare mil-spec barrel off the bench and hit it with the calipers for me?  

Thanks!
Scotty

http://scotty.geostrachan.com/hosted/ar15_fh.gif