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Thread: Will my 50BMG blow up?

  1. #1
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    Will my 50BMG blow up?

    Can anyone here validate my numbers on barrel size over the chamber on a 50BMG??

    I recently blithely turned down the barrel of a 50BMG to 1.4 over a portion of the chamber, now I'm questioning my numbers simply because I can't find any evidence anyone's ever done this before. My numbers tell me that I'm safe clear down to 1.3 over the entire chamber but my math skills are barbaric.

    Chamber diameter .7 in affected area

    I'd like to be comfortable to 100,000psi

    Lilja CM bbl

  2. #2
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    Quote Originally Posted by alinwa View Post
    Can anyone here validate my numbers on barrel size over the chamber on a 50BMG??

    I recently blithely turned down the barrel of a 50BMG to 1.4 over a portion of the chamber, now I'm questioning my numbers simply because I can't find any evidence anyone's ever done this before. My numbers tell me that I'm safe clear down to 1.3 over the entire chamber but my math skills are barbaric.

    Chamber diameter .7 in affected area

    I'd like to be comfortable to 100,000psi

    Lilja CM bbl
    Are you going to use hankies for cleaning patches?

  3. #3
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    Quote Originally Posted by alinwa View Post
    Can anyone here validate my numbers on barrel size over the chamber on a 50BMG??

    I recently blithely turned down the barrel of a 50BMG to 1.4 over a portion of the chamber, now I'm questioning my numbers simply because I can't find any evidence anyone's ever done this before. My numbers tell me that I'm safe clear down to 1.3 over the entire chamber but my math skills are barbaric.

    Chamber diameter .7 in affected area

    I'd like to be comfortable to 100,000psi

    Lilja CM bbl
    I don't know the answer Al but as you know, peak pressure occurs some distance down the bbl. Those small pencil bbls on light hunting rifles in huge and very overbore cartridges make me both wonder and gives me confidence at the same time. Lol!

  4. #4
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    Quote Originally Posted by antelopedundee View Post
    Are you going to use hankies for cleaning patches?
    traditionally I've used cobs but if you say hankies are better.....

  5. #5
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    alinwa,

    Check SAAMI specs, or the TM, since the 50BMG does NOT operate anywhere near 100,000 psi.....w/o a Bbl obstruction!

    You're basically dealing w/ a 30-06 case enlarged/magnified X 2.
    Last edited by Dans40X; 06-04-2022 at 05:44 PM.

  6. #6
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    Quote Originally Posted by Dans40X View Post
    alinwa,

    Check SAAMI specs, or the TM, since the 50BMG does NOT operate anywhere near 100,000 psi.....w/o a Bbl obstruction!

    You're basically dealing w/ a 30-06 case enlarged/magnified X 2.
    I have no idea how I can use SAAMI specs nor "the TM" to answer my quite specific question.

  7. #7
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    Quote Originally Posted by alinwa View Post
    traditionally I've used cobs but if you say hankies are better.....
    Old Sears catalogs too. What size patch does a .50 cal take?

  8. #8
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    Quote Originally Posted by alinwa View Post
    I have no idea how I can use SAAMI specs nor "the TM" to answer my quite specific question.
    You might check with Hodgdon about typical working pressures for .50 BMG loads. They may be able to answer your specific questions. Would any of the barrel makers have an idea.

  9. #9
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    Quote Originally Posted by antelopedundee View Post
    You might check with Hodgdon about typical working pressures for .50 BMG loads. They may be able to answer your specific questions. Would any of the barrel makers have an idea.

    I know perfectly well my working pressures.....stock doesn't interest me. I don't buy a 66 Nova to restore the L6 motor in front of a 2-speed powerglide.

    Building a 50BMG like everybody else does it interests me slightly less than buying a crate of SKS's and some cases of steelcore ammo

  10. #10
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    ran this by one of my kids in church this morning, interesting take......

    "since your chamber diameter=.700, your radius (working pressure) = .350" .... "And since your barrel diameter is 1.400 minus .700 the thickness is also equal to .350 therefore it's 1:1, ie chamber pressure = burst pressure"

    So basically the tensile strength of the steel EQUALS the allowable pressure before she ruptures! And since Lilja uses a 4140 variant with a tensile strength of 95,000 it looks as though even a proof load should be OK (I'll probably be running this gun at 55-62,000psi)

    But not the safety margin my initial figures indicated because I'd mistakenly used a figure of 120,000psi for my tensile strength.

    I guess this one WON'T be a hotrod I'll stick with a full-sized shank for my "test to destruction" experimentation

  11. #11
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    The 'trick' to outrageously high chamber pressures is to have a 'built up' barrel and chamber area.
    The outer layers are heater, the inner chilled, and then they are pressed together.
    Think large gun artillery barrels.

    You can often see the steps where the outer compressive layer ends.
    If you can develop 50.000 to 100,000 PSI of compressive loads the inner metal has that much more 'room' in the load department.
    Never seen in done on anything below .50 cal though.
    Last edited by brickeyee; 07-23-2022 at 03:31 PM.

  12. #12
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    Ohhh, I gave up with the numbers and fired it over a month ago


    https://www.youtube.com/watch?v=C0jCEu8yVyY&t=58s

    after this I ran 'er up to over 80,000psi

  13. #13
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    bolt thrust

    I'm sure you're aware but bolt thrust of a 50 BMG, at EQUAL PRESSURE is nearly double that of a ppc family cartridge, fwiw. I know you're burning a pile of powder, but pressure is pressure. Bolt thrust is a different ballgame with different case heads. Just saying, don't lose sight of that. No pun intended.

  14. #14
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    Quote Originally Posted by mwezell View Post
    Bolt thrust is a different ballgame with different case heads. Just saying, don't lose sight of that. No pun intended.
    This ^^^^^^

  15. #15
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    Bolt thrust in M2

    We had many M2 machinegun barrel extensions break due to excessive bolt thrust. This was caused by soldiers lubing the ammo before firing. The brass was sticking in the chambers because it was softer than standard design. Some solider had the idea of lubing the ammo to stop the sticking. The soft brass came about from a second source ammo producer using a different case forming procedure, thus producing ammo with a softer case than the LC GI standard as it wasn't work hardened enough in the case forming process. This forced a brass hardness profile to be added to the TDP. Be aware of any older brass with an IMI or PMJ headstamp. It was also sold under other private party names. Use LC or WCC headstamp brass for consistent brass hardness.

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