Retrieved: 12/14/2013

I hate threading to start with, and I've cursed the guys who designed rifles with square threads, which are even more annoying to cut.

I just realized why some rifles have square threads.

On a normal 55 or 60 degree thread, the barrel torque, due to the thread angle, tries to compress the barrel and expand the breech ring. The V thread acts as a lateral wedge trying to split the breech as well as a longitudinal wedge tensioning the barrel in place.

This means there's a considerable "hoop stress" on the receiver just sitting there, before a shot is fired. Particularly if the barrel has been torqued down by enraged gorillas or certain benchrest shooters.

The square thread, on the other hand, runs longitudinally only, with the thread face at 90 degrees. When you torque a square thread, its tension is longitudinal only; no extra load on the breechface. Also, no mixed compression and shear load on the threads; the load is in shear only, at the base of the thread. So the breech ring can theoretically be smaller and lighter to have the same strength as a V thread ring.

In practice, it's probably not an issue, and V threads sure are easier to cut...

TRX
August 31, 2013

The P-17 Enfield blueprints for the barrel have a note, "threads may be hobbed." I *think* that means they might be cut by live tooling, like "milled" threads on a CNC.

Hmm, I need to see if it might be possible to cut the threads with a slitting saw held at an angle... of course, you still have to single-point the receiver threads, or use a square tap.