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    • #27884
      Harter
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      I’ve read several articles threads etc and for the most part it was recycled 40s vintage information. So today we can practically put a full balistics lab in a shirt pocket , ok fine a moderate brief case for less than the price of a 5 yo car.

      1 reasoned argument of bolt thrust states that if a cartridge makes 5 kpsi or 65 kpsi that is all the load there is on the bolt as the bolt is only supporting the closed end of the cartridge.
      2 reasoned argument states that a tapered case will have greater bolt loads than a straight case because the case will try to slide with the expanding taper.
      3 reasoned argument states that what goes forward also pushes back.

      Ol’ Ackley greased some cases and wrote that within reason the brass would stick to the chamber walls unless there were a complete loss of contact ,like hydroplaning on the highway only with oil brass and steel . He did presumably 100s of tests ,but he didn’t have strain and crush gauges to take all of the measurements that Joe can today. He barely had a chronograph available.
      Also if this is correct then there is no reason that a 3″ 20 ga can’t be a platform for any number BP cartridges. (Other research says only a complete lunatic would build a BP only 45-120 on a 12 ga NEF action and even Colts are dangerous)

      Taper vs straight. Well does a 7.62×54 generate more load than a 30-06′ ? I have no idea . Under the premise that only case pressure is on the bolt or breach face then case taper and shoulder angle are moot points . If case taper is a factor then the old Nitro doubles must be be built hell for stout and a 300 H&H should develop way more load than a 300WM or even Weatherby. I would think if shoulder matters then a 22-250 should generate more than a 300 Savage and a 7mm-08 a lot more than a 358 Win . If momentum is the major factor then certainly the 358 should exceed the 7-08 by a wide margin.

      It seems to me that the actual bearing areas are is a major player . If you really look at a modern shot gun there not really much of a locking point to support 2 oz of projectiles reaching 1300 fps . Plastic isn’t all that sticky compared to brass so the lugs and or breach face have to be carrying most of the load .

      Does the bolt/breach face merely close the the chamber and keep the cartridge head flat or does it support the total load of slip, inertia and pressure ? How much difference does taper , shoulder shape make if any ?. Does a straight or nearly straight case have a lesser or greater load than a steep shouldered cartridge? Is there a source of imperical data that validates which factors make the most difference in actual load on any breach face.

    • #27888
      goody
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      Interesting question that I used to ponder on. No answers from me but interested in others thoughts also.

    • #27897
      Sgt. Mike
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      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.

      CARTRIDGE CASEINSIDE DIAMETER (HS)
      222.300″
      PPC.370″
      308.385″
      MAGNUM.420″
      378 WBY MAG.500″
      50 BMG.680″

      Bolt Lug Shear strength Unless a cartridge case undergoes a complete head separation upon firing, the side walls of the brass case will stick against the chamber walls. Under some circumstances they may absorb as much as half the thrust. Case walls or a chamber that are oily will reduce this friction. So the action designer will not take this aspect into consideration when designing the lugs to more closely simulate a complete case head failure.
      Also, in the formula that will follow the calculated strength value will be reduced by half, adding an additional safety margin of two. This is in keeping with generally accepted engineering practice for suddenly applied loads.
      The formula for calculating lug shear strength is:
      LS=(L*LL*NL*YS)/2
      Where:
      LS is the calculated lug shear strength
      L is the length of the arc segment
      LL is the axial length of the lugs
      NL is the number of lugs on the bolt
      YS is the yield strength of the material the lugs are made from
      As mentioned, this number is then divided in half for a safety margin of two.

      The only thorn in this formula is the length of the arc segment. We need to determine the total area in shear. Because the root of the lugs are joined to the bolt body on a radius, the length of the radial segment needs to be calculated. The formula for this calculation looks like:
      L=.01745*R*ANG
      where:
      R=the radius of the bolt body
      ANG=arccosine of the angle of the segment or
      ANG=arccosine(x/SQR(1-x^2))+1.5708
      where:
      x=1-(R-.5*SQR(4*R^2-W^2))
      where:
      R=the radius of the bolt body
      W=the width of a bolt lug
      The type and hardness of the steel the bolt is made from is important too. This part of the equation falls under the yield strength input. The yield strength is the maximum amount of pressure the steel can take without becoming permanently deformed. Up to this point it will return to its original condition. Since 4140 type chrome-moly is probably the most common type of steel used in bolts, here is a list of the yield strength for this steel at various Rockwell `C’ hardness values.

      ROCKWELL ‘C’ HARDNESS4140 YIELD STRENGTH PSI
      2083,500
      2287,000
      30135,000
      34148,750
      37159,000
      42178,000
      46195,000
      49211,000

      Bolt Flex

      Bolt flex or spring is an interesting measure that is easily calculated using the numbers we have already determined for area in shear and bolt thrust. Another number that we will introduce into the equation is the shear modulus of elasticity. For steel this number is 11,500,000 pounds and is a constant. It does not change regardless of the type of steel being used or the heat treatment of the steel.
      The equation looks like:
      FLEX=THRUST/(SA*11500000)
      where:
      SA=the shear area of the bolt lugs or:
      SA=L*LL*NL
      Or more simply put, the area in shear for all lugs is multiplied times the shear modulus, and this number is divided into the thrust. Often the resulting number will be in the .001″-.002″ range. This explains why it becomes necessary to bump the shoulders of brass cases back after a few reloadings. New brass is elastic enough that it will return to its original shape, but with progressive loadings the brass becomes more plastic until it does not return to its original form at all. Cases become sticky, bolt lift more difficult and eventually the cases have to be replaced. With very high pressure loads this can happen on the first firing. Also, the lug abutments in the receiver are set back a small amount too, compounding the problem. In fact it is possible for machining operations on the action (such as scope base screw or guard screw holes, magazine cutouts, and feed ramps) to weaken it to the point that the lugs have more strength than the action. In single shot bolt actions this would not be the case, but with light weight repeaters this is entirely possible and quite probable, at least with the bottom side abutment.
      So from a bolt flex standpoint we can see that the more lug area in shear, the less case stretch we will have. And this explains why higher pressure loads cause case stretching and sticky bolt lift. The obvious ways to increase shear area are: increasing the number of lugs, lengthening the lugs, or making them wider. Increasing the minor diameter of the lugs would help too, but to a lesser extent.

      By: Daniel Lilja (http://riflebarrels.com/a-look-at-bolt-lug-strength/)

    • #27899
      Sgt. Mike
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      Hope that helps your questions.
      I figured that the easiest way to explain your question was to cut and paste this article and allow you to calculate it on your own.

      7.62x54R Chamber pressure 56,565 psi
      Use the inside dimension of a magnum (.420″) unless you section one to get better or more accurate dimension

      THRUST=AREA* 56565 aka CPSI Where:
      AREA=3.1416*(.420/2)^2

      The 30-06 would use the same as the .308 (.385″) in the above post. Max chamber pressure is 60,000psi

      THRUST=AREA* 60000 aka CPSI Where:
      AREA=3.1416*(.385/2)^2

      AreaChamber PSIBolt Thrust (psi)
      30-060.116415915600006985.0
      7.62X54R0.13854456565657836.8
    • #27903
      Sgt. Mike
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      I did not do a reduction of bolt thrust in the above calculation (which means the case did not expand to the chamber) which would be 50% to 1/3 dependant upon case body taper and case construction. Steel cases do not expand then contract to the level that cartridge brass does, which reduces the bolt thrust more than a steel case

    • #27914
      Goodsteel
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      One aspect of this is the direction of force. Fact is, there is more pressure on the sidewalls of the case, than on the bolt nose. This is both because of the way the cartridge is shaped and the fact that the bullet relieves the pressure in the linear direction.
      Whether the case makes grip on the walls of the chamber, or the bullet relieves pressure doesn’t really matter. The fact is, it’s the BARREL that absorbs the lions share of the pressure and the force on the bolt face is less. The straighter the walls of the case get the less pressure there is on the bolt nose/recoil lugs. With an extremely tapered cartridge, there is a certain amount of additional pressure that is exerted rearward while these things are taking place. Often on a soft action, the lugs are set back little by little till (God forbid) they break loose. Often, and early warning is that the bolt becomes stiff to work because as the lugs shear off they rotate the leading edges inward which pinches the bolt and impedes function slightly.

      Another thing that few people take into consideration is the size of the cartridge vs. the PSI that it functions at. It’s just like the cubic displacement of an engine. For a given PSI, you get more thrust from a larger ignition chamber, and in this case, that translates to bolt thrust.
      This was known back in the middle of the last century and is evidenced by the design of certain cartridges like the 500 Jeffery. This cartridge was too large for the Mauser rifle it was designed for, but they made the side walls very straight and used a rebated rim just to get it in the bolt face. The straight walls made it so the lugs on a good Mauser could take the increased force created by the larger case.

    • #27915
      lar45
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      Harter;n6893 wrote: I’ve read several articles threads etc and for the most part it was recycled 40s vintage information. So today we can practically put a full balistics lab in a shirt pocket , ok fine a moderate brief case for less than the price of a 5 yo car.

      Also if this is correct then there is no reason that a 3″ 20 ga can’t be a platform for any number BP cartridges. (Other research says only a complete lunatic would build a BP only 45-120 on a 12 ga NEF action and even Colts are dangerous)

      It seems to me that the actual bearing areas are is a major player . If you really look at a modern shot gun there not really much of a locking point to support 2 oz of projectiles reaching 1300 fps . Plastic isn’t all that sticky compared to brass so the lugs and or breach face have to be carrying most of the load .

      Does the bolt/breach face merely close the the chamber and keep the cartridge head flat or does it support the total load of slip, inertia and pressure ? How much difference does taper , shoulder shape make if any ?. Does a straight or nearly straight case have a lesser or greater load than a steep shouldered cartridge? Is there a source of imperical data that validates which factors make the most difference in actual load on any breach face.

      If you start with the NEF action that is made for the handi rifle and chambered for something in the 30-06 class, then you would have a stronger platform to start with.

      Ed Hubbel has done some work with the NEF chambered in the 12ga from Hell, 12gaFH, I think they ran the pressures up to around 30k, don’t take my word on it look it up yourself just to make sure.

      I would see no problems with doing a 45-120 with BP or Smokeless loads.

    • #27931
      Doc Highwall
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      Area X Pressure = Force.

      Say you have 100,000 psi of pressure, but the area that it is applied to is only .100, then the force being held is 1000 or 1/10 of the 100,000 psi.

      Conversely if you have two cartridges where one has twice the surface area of the other, the one with half the surface area can be loaded to twice the pressure and still have the same force applied to the steel.

      Have you ever seen where they heat up a 55 gallon drum with a little water in it making steam, then take it off the heat and cap it and cool it down. This is what happens with only 14.7 psi or one atmosphere.

      https://www.youtube.com/watch?v=JsoE4F2Pb20

    • #27933
      lar45
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      Doc Highwall;n6948 wrote: Area X Pressure = Force.

      Say you have 100,000 psi of pressure, but the area that it is applied to is only .100, then the force being held is 1000 or 1/10 of the 100,000 psi.

      100,000 x .1 = 10,000

    • #27937
      goody
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      Good job, I now know more than I did before! I do love learning, they say it helps your brain in the long haul!!

    • #27938
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      Exploring farther. Is you have a cartridge that doesn’t reach “grip ” pressure it would apply full load to the breach face wouldn’t it . Approximated a lightened 38short is more abusive than a typical 357 …… food for thought.

      Thanks for the guidance , I am now fully equipped to be really dangerous. 🙂

    • #27950
      Doc Highwall
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      What I meant in post #9 was 1/100 of a square inch. This if it was a square area would measure .100″x .100″

    • #31484
      popper
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      2 reasoned argument states that a tapered case will have greater bolt loads than a straight case because the case will try to slide with the expanding taper.

      Uh, not much – pressure is always applied perpendicular to the surface & the small taper angle(?/90) energy is mostly lost (dissipated) in case wall friction & case/barrel expansion. Taper is just there for extraction ease only. Now the shoulder on a case does increase bolt thrust, just like the boolit. Internal case ‘head’ is 60% of external, IIRC 30% of powder energy is in expanding & heating of chamber, >60% goes down the barrel so actually very little is left for the bolt head.

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