I have a Zastava M85 that came with a barrel chambered in 5.56/.223. I wish to add a barrel chambered in. 300 BO. Unfortunately, I also have a South Bend Heavy 10 lathe and nothing but Imperial thread gearing. The Zastava action is threaded in M22x1.5 threads. I have measured it and verified it to be so.
As a work-around, due to the limitations of my lathe, I am considering using a pre-threaded M22x1.5 fitting and internally threading it to receive a barrel stub which my South Bend is capable of producing.
My question is, how to calculate the barrel wall thickness necessary to contain the 50,000+ cup pressure?
Is this the hard way to do this? Absolutely. Would it be easier to find someone to thread and perhaps even chamber the barrel for me? Absolutely. But that's not what this is about. I'm out to learn a skill, not learn where to farm problem work out to.
If anyone can assist with the calculations, I'd appreciate the replies. For the nay-sayers, flame suit on. Do as you see fit.
Thanks.
Interesting question. I have to say up front, that this does not seem like the best way to attach a barrel. However, at your own risk, here are my thoughts on how to do the calculations. What you need is a thick wall cylinder formula. You will find many on the internet, but they are complicated. At this link you will find a calculator. I have absolutely not done any checking of the formulas they are using or any verification of the accuracy of the results.
http://www.engineeringtoolbox.com/stress-thick-walled-tube-d_949.html
What I did do is test the calculator with some estimates of a Remington 700 barrel chambered in say a 7mm RM. The thread Remington uses is a 1 1/16 x 16, with what I think gives about 0.988" as the minor diameter. The case is about 0.510" in diameter at the head. Here is what I put into the calculator:
Inside Pressure: 61000 psi
Outside Pressure: 0 psi
Inside radius: .255" (.510/2)
Outside radius: .494 (.988/2)
Radius to point in the wall: .255" (the inside diameter will be the high stress point and can't be above yield of the material)
This gives a hoop stress of 105,316 psi which is much higher than what I expected it would be.
I did the same thing with what you are proposing to do, but with the .300 BO and a pressure of 55,000 psi.
Inside Pressure: 55,000 psi SAAMI ( I see CIP uses a much higher 62,000!)
Outside Pressure: 0 psi
Inside radius: .188"
Outside radius: .354" (my guess as to what you might end up with as a radius to the minor diameter of the thread)
Radius to point in the wall: .188"
Hoop stress: 98,212 psi - That is high, but lower than what the Remington 700 7mm RM works out to be. With the 62,000 psi CIP pressure the stress is much higher at 110,000 psi, and significantly higher than the 7mmRM example. Danger!!!
You would next have to compare that to the yield stress of the barrel material you plan to use. You really do not want to be close to yield of the material.
I hope that helps some, but you are on your own. This was just a quick look, and I have no idea if that calculator is right or not. Interesting problem aside, I would get the barrel threaded to the proper thread and be done with it... I believe it would give you an outside radius of 0.392", and a stress of 99,000 psi, even at 62,000 psi chamber pressure. And that .392 makes me think I was a little optimistic in thinking your bushing solution would give you an outside radius of .354". Could be smaller. Hope that helps some with your dilemma.
