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Old November 11, 2012, 11:15 AM   #7
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Join Date: May 17, 2012
Posts: 1,084
I finally figured out I need an elk thumper like I need a bloody nose.
HA! I don't own anything more powerful than a 30-06, so I'd like at least one representative from the Big Bullet family (though already having an '06, I'd scarcely need anything more)

Great responses, ya'll; much food for though here (so please bear with my ramblings).

Bart, all the numbers are in psi, but they are from Wikipedia, so take them with a grain of salt if they don't match what you already know

One thing that is missing in the bolt thrust calculations is the friction of the case on the chamber.
I purposely neglected this because friction loads cannot be relied upon (due to oil, dust, etc.) to always be there. That's how the FAA treats the issue with regards to airplane design, so I figure that's how I should approach the issue with guns. Think of it as an "extra" extra saftey factor
I would expect it to be a considerable proportion.
In a high-pressure small-bore necked cartridge, that makes sense, due to the high case-body surface area and small bolt face. But in a 45+ caliber, there's so much more bolt face area relative to the surface area of the cartridge, and the lower pressures won't push outward as hard. It seems like a diminshing factor for these chamberings (whether or not it is still consequential, I can't say, though; I haven't seen any numbers on the subject)

A rule of thumb is that single-lug bolts not be loaded for chamber pressures above 40,000 psi
The Steyr M95 has two large lugs ~.3"x.3" and about .1" tall, which rotate 90degrees just like most other bolt actions. Large enough that the contact surfaces should bear-out before they'd shear off, I believe (haven't calculated that yet, since I don't know the yields on this steel). I wonder if that rule is for prevention of brittle fatigue fracture; high pressures are percussive, causing microcracks to spread a bit faster than if they were loaded more gently. All rifles have to deal with this, but a single-lug has no backup--making any fracture unacceptable, thus the lowered pressure.
Safety testing is generally 50% or more above this.
I would like to attempt a proof load of some sort before shouldering the thing; I'd have to look into how one goes about calculating the overload, though, then buy a wooden clamp and some kite string

Bolt thrust isn't purely a function of square inches and psi. The type and condition of the chamber and any source of lubrication between it and the case is a big factor.
Interesting website; FEM is always cool for demonstrating concepts/theories. It appears lubricating the case (a supposed no-no) reduces brass-loading at the expense of increased bolt thrust (makes sense). But the .243Win is a small bore, long and narrow cartridge, with a (relatively) high 55000psi operating pressure. A 45-70 is at the opposite spectrum, so I wonder if those effects still have the same impact (or something else comes into play to mess with bolt loads)

I hoped that by neglecting friction effects I can get conservative estimates, and that if my "new frictionless cartridge load" is lower than the "old frictionless cartridge load" I'll be safe. Like I said, I'm not an expert in this field so I can't trust my assumptions at face value--which is why I'm asking around here . If I was to rechamber the Steyr to 50 AK, the failure would probably occur in the stock before anything else, though

"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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