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Old November 10, 2012, 11:54 PM   #1
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Join Date: May 17, 2012
Posts: 1,085
Chamber Pressure in Regards to Cartridge Selection

In the course of research for my mad M95 project, I've seen much made of "chamber pressure" as being the limiting factor in whether a load is safe in a given action. It would seem to me that the force applied to the weak link in the action (lugs, I would assume) is the limiting factor, but pressure alone misses the effects of increased area on bolt thrust when the bolt face is opened for a bigger cartridge. Or is some other factor dependant only upon pressure what the smiths are paying attention to?

Not that I would attempt something so insane (my shoulders are gimped as it is), but I ran the numbers on 50 Alaskan in the Steyr M95 simply because the cartridge could physically fit on the bolt-face and magwell:

50 Alaskan
From Wiki (not the best source for load data )
Pressure: 40,000psi
Base Diameter: .553in (my assumption for thrust area on the bolt-face)
-Area: .240in^2
Calculated bolt thrust: 9605.5lb (ouch)

8x56R, the "original" chambering
Pressure: 51,500psi
Base Diameter: .494in
-Area: .192in^2
Calculated bolt thrust: 9868.9lb (ouch-ier)

8mm Mauser (Greeks/Yugos rechambered their M95s to this)
Pressure: 57,000psi
Base Diameter: .470in
-Area: .174in^2
Calculated bolt thrust: 9887.3lb (hope those lugs had a +20lb safety factor)

45-70(the round I plan to rechamber to)
Pressure: 43500psi(taken from limit for 450Marlin, since SAAMI wussed-out the 45-70)
Base Diameter: .505in
-Area: .200in^2
Calculated bolt thrust: 8711.2lb (yawn...yeah right)

Are my math/assumptions seriously off, or are all these loadings within/near the stresses of the factory-tested 8x56R? I understand this "analysis" neglects the duration of the peak-pressure spike. But still, the Alaskan load at 3346ft-lbf causes 260lbs less bolt-thrust than the 2504ft-lbf Mannlicher? What am I missing here?! All indications are that recoil (determined by bullet momentum rather than bolt thrust, I know) from the 50 would surely kill me, whereas the 8x56R will merely maim me

"I don't believe that the men of the distant past were any wiser than we are today. But it does seem that their science and technology were able to accomplish much grander things."
-- Alex Rosewater
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