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Thread: Bolt Thrust & Pressure?

  1. #1

    Bolt Thrust & Pressure?

    Hello all.

    So I picked up a NOS Lyman 358627 which got me to thinking.
    Can I run this in my Uberti 1873? I know it’s built to handle a lifetime of 357 magnum and there’s guns that have done just so. No manufacturer in their right mind would advise using their rifle with loads that would wear the gun down in no time flat. Never mind possibly causing damage that would result in a law suit.
    Which got me to thinking about pressure vs bolt thrust.
    I know the 357 magnum is loaded to more than double the pressure of a 45 Colt. However how does that translate to bolt thrust? The pressure is held within the case and the chamber right? Bolt thrust should be the main factor when it comes to damage to a toggle link action right?
    So how does a 255 grain 45 colt bullet loaded to 14k psi compare to a 215 grain 357 mag bullet loaded to say 30k psi? Vs a 158 grain 357 loaded to 30k psi?
    Am I thinking about this properly? Which has more bolt thrust?
    I know that Uberti also makes 44 mag 1873 rifles now. Though they only appear in blued actions and not case hardened which has always made me wonder if it’s a different steel or heat treat used for those actions alone? I’ve searched around a ton and can’t find any info to prove or disprove that.

    All my research shows that bolt thrust discussions are one of the top 3 ways to start a bar fight. Some people don’t believe it exists. Some do.
    So if it doesn’t then does it matter what bullet weight I shoot as long as I keep it under the 357 magnum pressure limits?

    That being said I think I would stick to some light plinking loads with this bullet in the 73 because I really just plink at steel anyway and the extra weight should ring it like a bell.
    Thanks for looking all!
    Last edited by Michael J. Spangler; 02-08-2020 at 11:25 PM.

  2. #2
    Boolit Master
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    Keeping things basic, the bolt thrust is the perk pressure times the area of the case at the web. The case is basically a piston operated on by the gasses at chamber pressure. A 357 case is .375ish so area is roughly .110 sq inch. A 45 Colt case is about .48(too lazy to look it up) so .181 area. 14,000 pressure on that .180 sq inch gives 2533 pounds bolt thrust. The 357 at 30,000 ( bullet has no effect) gives 3300 lbs thrust. This ignores friction of the brass against the chamber walls ect, but generally shows the relationship.

    Note, I was doing the math as I typed. Rereading your post , I think you are wondering if bullet wat has an effect, no not on bolt thrust. Revolvers are another story there.

    Those people who don’t think it exists are why contender frames get stretched. It exists, granted the case grabbing the chamber alters the value , some, but that is how brass stretches in flexy actions with correct headspace. Lots more to that topic, but basically ..
    Last edited by rking22; 02-08-2020 at 11:34 PM.
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  3. #3
    Boolit Grand Master



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    Bolt thrust is determined by the inside area of the cartridge case head not the rim diameter or body diameter. Formula's here https://riflebarrels.com/a-look-at-bolt-lug-strength/

    Bolt Thrust
    Bolt thrust is easy to calculate. Only two inputs are required. They are peak chamber pressure in PSI and as mentioned, the inside area of the case head that the gas pressure can work on. The formula then is:

    THRUST=AREA*CPSI Where:

    AREA=3.1416*(HS/2)^2

    HS=the diameter of the inside of the case head.

    I sectioned some cases and measured the inside diameters and found that they were as follows:

    CARTRIDGE CASE INSIDE DIAMETER (HS)
    222 .300″
    PPC .370″
    308 .385″
    MAGNUM .420″
    378 WBY MAG .500″
    50 BMG .680″
    The thrust is measured in pounds per square inch.
    Last edited by M-Tecs; 02-09-2020 at 02:00 AM.
    2nd Amendment of the U.S. Constitution. - "A well regulated Militia, being necessary to the security of a free State, the right of the people to keep and bear Arms, shall not be infringed."

  4. #4
    Boolit Grand Master Texas by God's Avatar
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    Bolt thrust is proportional to case head size. Think of the original T/C Contender. They could chamber it in .223 Remington which is a high pressure round; but not the .308 Winchester because the wider base of the .308 transfers more bolt thrust to the standing breech.

  5. #5
    Thank you for the reply.

    So the bullet weight can change without a change on bolt thrust.
    So I guess the big question is if bolt thrust is the main factor in the wear of a lever action?
    I’m used to thinking about semi autos with their recoil springs and heavier bullets battering frames. That shouldn’t be the issue with a locked action correct? There’s no movement of the action that needs to be balanced out with springs. So there’s no increase of frame battering due to heavier bullets.
    So bullet thrust which is based off case pressure and diameter should be the main factor?
    So the 357 and the 45 colt appear to be quite equal in this respect?

  6. #6
    Boolit Man
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    Cool

    FWIW, bolt thrust is expressed in pounds, not psi. Pressure is expressed in psi. But bolt thrust is not the only concern, heavy recoil can cause increased battering of fragile actions. I donít know the effect on the OPís rifle, but it is a consideration. I donít know if that bullet will feed reliably in the M1873. I do know that a 215-grain bullet loaded to max pressure will increase recoil.



    .

  7. #7
    Boolit Grand Master



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    Quote Originally Posted by Texas by God View Post
    Bolt thrust is proportional to case head size. Think of the original T/C Contender. They could chamber it in .223 Remington which is a high pressure round; but not the .308 Winchester because the wider base of the .308 transfers more bolt thrust to the standing breech.
    Bolt thrust is determined by the inside diameter of the case. That is what prevents us from rebating a .308 rim to .223 rim. In both cases the bolt thrust is the same, however, the PSI in the contact area will be greater with a smaller area.
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  8. #8
    Boolit Grand Master



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    Quote Originally Posted by Michael J. Spangler View Post
    Thank you for the reply.

    So the bullet weight can change without a change on bolt thrust.
    So I guess the big question is if bolt thrust is the main factor in the wear of a lever action?
    I’m used to thinking about semi autos with their recoil springs and heavier bullets battering frames. That shouldn’t be the issue with a locked action correct? There’s no movement of the action that needs to be balanced out with springs. So there’s no increase of frame battering due to heavier bullets.
    So bullet thrust which is based off case pressure and diameter should be the main factor?
    So the 357 and the 45 colt appear to be quite equal in this respect?
    Bolt thrust should have zero impact on wear of any fixed breech firearm unless the design limits are exceeded. Normally this would be bolt lug setback or in the case of guns like the AR broken lugs or in the case of the 73 linkage compression.

    Uberti does chamber the 1873 in 44 Mag. https://www.uberti-usa.com/1873-rifle-and-carbine

    Some interest discussion here

    https://www.levergunscommunity.org/v...ic.php?t=38460
    Last edited by M-Tecs; 02-09-2020 at 02:30 AM.
    2nd Amendment of the U.S. Constitution. - "A well regulated Militia, being necessary to the security of a free State, the right of the people to keep and bear Arms, shall not be infringed."

  9. #9
    Boolit Master
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    Quote Originally Posted by M-Tecs View Post
    Bolt thrust is determined by the inside area of the cartridge case head not the rim diameter or body diameter. Formula's here https://riflebarrels.com/a-look-at-bolt-lug-strength/

    Bolt Thrust
    Bolt thrust is easy to calculate. Only two inputs are required. They are peak chamber pressure in PSI and as mentioned, the inside area of the case head that the gas pressure can work on. The formula then is:

    THRUST=AREA*CPSI Where:

    AREA=3.1416*(HS/2)^2

    HS=the diameter of the inside of the case head.

    I sectioned some cases and measured the inside diameters and found that they were as follows:

    CARTRIDGE CASE INSIDE DIAMETER (HS)
    222 .300″
    PPC .370″
    308 .385″
    MAGNUM .420″
    378 WBY MAG .500″
    50 BMG .680″
    The thrust is measured in pounds per square inch.
    Thrust is pounds

    Pressure is pounds per square inch

    Area is square inches

    Thrust in Pounds = Pressure in pounds per square inch * Area in Square inches

    If you are treating the case as a piston pushing on the bolt then the inside diameter of the chamber at the case's expansion ring is the proper cross sectional area for this calculation. This assumes the case slides back and does not stretch. The brass does go into tension but the amount is hard to predict. It depends on the roughness of the chamber, the amount of lubricant on the case or in the chamber.

    If the lock up is not tight and the case and bolt get a running start at the locking mechanism all bets are off. Impact loads are a lot different than static pressure loads. Thrust is a load, a force.

    All and all the bullet weight does not come into play. The bullet weight matters more when we are talking about recoil. Heavier bullets produce more recoil for a given muzzle energy.

    Tim
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  10. #10
    Boolit Master
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    Quote Originally Posted by Texas by God View Post
    Bolt thrust is proportional to case head size. Think of the original T/C Contender. They could chamber it in .223 Remington which is a high pressure round; but not the .308 Winchester because the wider base of the .308 transfers more bolt thrust to the standing breech.
    This is true for rimless rounds. The thrust to the breech in this case causes the frame to stretch either causing it to unlock or to crack at the hinge point. If you stretched the .300 blackout until it had the capacity of the .308 you could get .308 performance out of the Contender without breaking it.

    Tim
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    The pen is mightier than the sword - Edward Bulwer-Lytton

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  11. #11
    Boolit Bub 1006's Avatar
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    High pressure 9mm/38 Super show evidence of bolt thrust effect on 1911 breech faces. How would a semi-auto work if no bolt thrust existed, in a non-gas operated gun?

  12. #12
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    Quote Originally Posted by 1006 View Post
    High pressure 9mm/38 Super show evidence of bolt thrust effect on 1911 breech faces. How would a semi-auto work if no bolt thrust existed, in a non-gas operated gun?
    The function of the M1911 and other similar Browning type actions operate on recoil impulse not bolt thrust. The bolt thrust has nothing to do with the operation/functioning of such actions.
    Larry Gibson

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  13. #13
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    Here is a quick and dirty Excell sheet for bolt thrust
    http://www.lsstuff.com/misc/bolt-thrust.xls

    I have it setup for the case head size, but the more correct way is the inside of the case. Measuring the base of the case will get you close, but if you want to know exactly what it is, then section a case and measure it.

  14. #14
    Quote Originally Posted by lar45 View Post
    Here is a quick and dirty Excell sheet for bolt thrust
    http://www.lsstuff.com/misc/bolt-thrust.xls

    I have it setup for the case head size, but the more correct way is the inside of the case. Measuring the base of the case will get you close, but if you want to know exactly what it is, then section a case and measure it.

    Thanks!

  15. #15
    Boolit Master

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    Quote Originally Posted by Texas by God View Post
    Bolt thrust is proportional to case head size. Think of the original T/C Contender. They could chamber it in .223 Remington which is a high pressure round; but not the .308 Winchester because the wider base of the .308 transfers more bolt thrust to the standing breech.
    I tried to explain this once and some know-it-all here, he said bolt thrust had on effect on the breach face and seemed to think a 300 Winchester magnum would have been just fine in an original Contender.

    I'll keep checking back to see if he makes the same argument with you.
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