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I have calculated the state of stress in the Bat Machine Co. Inc. 8.5" action for the .408 CheyTac Cartridge with a 0.630" case head diameter. The calculation was done with the LS-DYNA FEA code. Two conditions were calculated. The Nominal condition is for a 60,000 psi pressure (hot load) and an intact cartridge case with an internal diameter of 0.540". The second calculation assumes the worst case of a ruptured cartridge case head (but still contained within the chamber) with 60,000 psi pressure over the full diameter of the 0.630" case head. The high strain rate that occurs when a cartridge is fired was not considered. Typically, steels have up to 10% higher yield strengths at higher strain rates and that extra strength was not included in these calculations since the high strain rate properties of these steels was not known.

Summary Table

  Maximum Effective Stress at the bolt lugs
(psi)
Maximum Effective Plastic Strain at the bolt lugs
(in/in)
Maximum Effective Stress on the action
(psi)
Bolt Face Axial Deflection
(in)
Action Axial Deflection
(in)
Nominal Condition Intact Cartridge 181,500 0.0020 119,900 0.0070 0.0020
Ruptured Case Head 185,400 0.0046 163,300 0.0095 0.0028
 

Nominal Condition 60,000 psi with intact cartridge.


The yield stress is slightly exceeded at the very corners of the bolt lugs. This very small amount of yielding at the corners of the bolt lugs is not a serious problem and will not change the bolt's position for subsequent firings at or below the same pressure.


The plastic strain is slightly above the 0.2% yield offset and only at the corners of the bolt lugs.


Close up of the plastic strain location. It only extends over a very small area. The remaining elastic areas will allow the bolt to lock up in the closed position with no permanent setback. The total area of the bolt contact area would have to yield to cause bolt setback.


The maximum effective stress in the action is 75% of the yield stress.


At 60,000 psi, the bolt face moves rearward 0.0070" (rounded to 2 digits). This amount of elastic axial displacement is nominal.


The displacement of the action shelf accounts for 0.002" of the total bolt face axial displacement above.

 


The worst case condition with a ruptured cartridge case head and 60,000 psi pressure over the full diameter of the of 0.630 diameter" case head.


The yield stress is exceeded by 5,400 psi at the corners of the bolt lugs. The very small amount of yielding at the corners of the bolt lugs would not be a serious structural problem and would not change the bolt's position for subsequent firings because most of the area of the bolt lug remains elastic and does not yield. 


The plastic strain is 0.46% and is localized to the corners of the bolt lugs. Most of the bolt lug remains elastic and there is no permanent bolt setback.


The action remains elastic and the maximum effective stress is 99% of the yield stress.


At 60,000 psi, the bolt face moves rearward 0.0095" (rounded to 2 digits).


The displacement of the action shelf accounts for 0.0028" of the total bolt face axial displacement above.


Another view of the bolt and the effective stress.

The same view as above with the bolt lug removed and showing the fringes of Maximum Shear Stress. Yielding in shear would occur when the shear stress exceeds ~1/2 of the yield stress or 90,000 psi. The Maximum Shear Stress (the red fringe area) at the bolt to lug junction is only 55,300 psi at it highest point or 61% of the yield shear stress even for the worst case loading.

Material Properties used in the calculation:


Rifle Bolt of 4140 steel heat treated to a hardness of 43-45 Rc


Rifle Action of 17-4Ph Stainless Steel 36-38 Rc

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