Re: Buttress style Bolt lugs
The reason that buttress threads are used for anything, firearm related or not, is because of the loading properties of that thread.
In a regular V thread the surfaces in contact create very large hoop stresses and "expanding" forces that try to stretch the ring open.
I have seen this style thread actually "jump" under high load cases in a test rotor on a helicopter. During a run up test the nut on the top of the rotor shaft actually jumped off the pressure plate on the top of the rotore by about a half of an inch. It was something that was not supposed to be a V thread, and as soon as that jump happened we scrapped the rotor shaft and had a new one made with buttress threads on it.
The production rotor shafts in the Chinook have a buttress thread on the top of them where the rotor hub nut goes on. This is sometimes refered to as "the Jesus nut" since if it fails, you're going to see Him.
The reason you would do this in a large bore cannon is the same reason. The available area for the firing pressures to push on in that case are massive, which creates enormous loads axial on the breech block. With a regular V notch thread it would create a much higher hoop stress state because not only the firing pressure will be there to expand the breech ring, but the axial force on the V threads will put about 70% of that axial force into the ring expansion as well, practically doubling the load carried in the hoop direction (tangential stresses).
In a smaller bore firearm you would want to use this as a way to have a very high mechanical advantage during the final closing stages of a bolt without causing the high ring stress state in the tenon area of the receiver.
