Machine shop or gunsmiths? I honestly wouldn't let a machine shop work on anything that wasn't purely cosmetic.
A gunsmith should have the equipment to centralize the thread properly, generally it is all part of the equipment you need.
KBK[/QUOTE
Im not trying to start an argument, but.....
Uh......"machine shops" built virtually every metal part in your gun. They made the molds for the plastic lower. The springs were most likely made by a spring manufacturing specialty shop.
I'm not taking anything away from 'smiths, but any machine shop with a competent lathe guy can cut a barrel thread. It's not rocket science. Its not gunsmith science. It's simple machining 101. My guess is that many 'smiths simply hand the barrel off to a local machine shop to do the work.
I doubt most gunsmiths have the proper equipment to correctly and accurately measure the concentricity ( or total runout) of the thread with respect to the bore diameter.
Also, a typical flash hider ( at least the ones i have seen in stores and gun shows) has an internal diameter such that you could easily allow .05" concentricity ( that's about two miles in the machining world) of your barrel thread and still have plenty of clearance for the round to pass thru.
$50 is a good and fair price to have the thread cut if you walk into a shop and tell them what you need..... And if you know what you're asking for.