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How, when and if a barrel can be threaded, and how we set it up on the lathe for threading.
First, a few basic questions. When can a barrel be threaded, and what type of thread should I cut?
In the US, the basic thread types are 1/2" x 28 and 5/8" x 24. The 1/2" x 28 is found on most 5.56 AR-15's, and the 5/8" x 24 is found on 30 caliber rifles. A more extensive list is here: Thread pitch guide
The two main types of threading you will see are these two:
This thread does not go all the way to the shoulder of the barrel, it 'fades away' to the major diameter of the thread. the major diameter is the MAXIMUM diameter of the thread, it's what you would start with before you start cutting threads. This type of thread is going to be stronger than one that uses a relief groove (see below). Note that a female thread WILL NOT be able to make it all the way to the base of the thread, unless the female thread has some relief (material cut away) where it will overlap the incomplete faded thread.
This is a 5/8" x 24 thread on a 300 blackout. Note that the base of the thread has a short area with no threading, at about the MINIMUM diameter at the base of the threads. A mating female thread can go all the way to the shoulder without binding. While this thread has less material at the thread root, and will not be as strong as a faded thread, it is compatible with any female thread, even if that thread does not have a relief.
For muzzle brake applications, the barrel threading is a lot less critical than for suppressors. Muzzle brakes are in general short, usually no more than 5 diameters long. Suppressors on the other hand are typically 25 or more diameters long. Alignment with the bore is critical. While crush washers are fine for muzzle brakes to allow alignment, THEY MUST NOT BE USED WITH SUPPRESSORS OR SUPPRESSOR MOUNTS that are also muzzle brakes. These devices that require alignment with the bore must be mounted with shims of uniform thickness.