Guys,
The most important thing in any thread engagement is not the number of threads but it is the radial strength of the female part. In a standard coarse thread nut there are three threads which is strong enough to break the bolt if both bolt and nut are of the same strength. A heavy nut is much stronger than a standard nut, yet it still has the same three threads - it is thicker across the flats to resist failure by dialation.
So, when we breech a barrel we must pay close attention to the thickness of the barrel breech wall thickness. For instance, if we use a 3/4 thread breech in a barrel that is 15/16 leaving a 3/32 wall - an overpressure could cause the breech joint to fail by dialation. The pressure expands the barrel diameter and decreases the thread radial engagement resulting in a much weaker joint, and a potential failure. So try to keep a breech plug thread that will give a stout wall thickness while still sealing the bore end on a shoulder. Does this make sense?
As for me, I make the breech plug 9/16 long, giving about 12 threads. That is 18th c style threads.
Jim
