Chamber wall requirements

Discussion in 'Advanced Gunsmithing & Engineering' started by Someoldguy, Jun 16, 2017.

  1. Someoldguy

    Someoldguy

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    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.
     
    Ggmac likes this.
  2. RonAKA

    RonAKA

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    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.
     
    Last edited: Jun 17, 2017
  3. Someoldguy

    Someoldguy

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    I suspected this was nearing the limitations of the barrel material. Looks like I'd better look for other options. Thank you for your reply.
     
  4. AndyA

    AndyA Silver $$ Contributor

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    Now this is just a suggestion for something to try. 1.5mm is awful close to 17tpi. I don't have access to a SB10 so I don't know if it can do that pitch, but you could try a stub and see if it threads into the receiver. If it wont do 17tpi, I wonder if you could rough on a 16tpi and finish with a M22x1.5 die.
     
  5. Ggmac

    Ggmac

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    I replied to your other post , now I understand and appreciate your approach .
    I was going to suggest using the old barrel and use that to make your adapter , I've done it a few times with old rolling lock actions .
    Low pressure , actually did a few heat shrink stubs and silver soldered a few .
    I've done a few Mausers with adapters but for testing , long story , I used 2 dif thread pitch to maintain barrel position but increase or decrease headspace .
    OK your problem : green mountain barrels has 308 1/8 twist 41v50 cvm
    It's a stronger steel , chrome vanadium moly . I've done some 338 lap mags on std sav receivers and tested ( the way the British did , oil chamber ) over 380 rds and no measurable change . I chambered a stub at the same time of building and can fit fired brass in stub . Simple but it works .
    Pm if any ?
    Gary
     
  6. Someoldguy

    Someoldguy

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    PM sent. Thank you.
     
  7. Someoldguy

    Someoldguy

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    I'm not sure if this will work or not, but I plan to look into it. Thank you.
     
  8. GenePoole

    GenePoole

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    I have a South Bend 13 and I have the metric gears. Well worth the investment. If your SB is in good shape and you plan to keep it awhile, you'd be best off to just bite the bullet and get the gears. You'll need to do metric threads eventually.

    M27x2.0 AIAX threads on a SB13 (AI top, mine bottom):

    [​IMG]
     
  9. Someoldguy

    Someoldguy

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    As I recall with the Heavy 10, i need the entire quick-change gearbox, and possibly the idler set as well. (?) Maybe even the lead screw . . . . I may be mistaken, it's been a few years since i looked into it and I've slept too many times since then to be certain.
    Anyway, the lathe is in good shape, i replaced the compound and apron nuts when i got it, along with all the wicks, but the metric gearing was cost-prohibitive at the time. I'll look again when time allows, but i imagine the situation has not improved.
     
  10. Ggmac

    Ggmac

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    Sent a pm
    AR barrel specs .810 odx16 tpi so 810 od minus the thread is in your ballpark .
    The same tenon is used for larger cases such as the 6.5 Grendel and 6.8 SPC .
     
    Last edited: Jun 17, 2017
  11. Shawnba67

    Shawnba67 Silver $$ Contributor

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    ri
    right now on ebay is a 22x1.5 die for $11 if you choose that route
    good luck and happy shooting
     
  12. Someoldguy

    Someoldguy

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    I'm thinking that given I have no need for doing any metric threading in the foreseeable future, apart from this project, that I will be time and money ahead to persue this option. Thank you for the tip!
     

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