Chamber Pressure in Regards to Cartridge Selection

[barnbwt]

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

TCB

[Scorch]

You got it right, bolt thrust is chamber pressure X cartridge head area. And yes, higher pressure cartridges really load the bolt lugs, if only for a short time. And most of the recoil comes from the ejecta (the mass of the burnt powder), not the weight of the bullet. So even if a cartridge has higher energy, if it operates at lower pressure, the ejecta are traveling slower when they hit the stationary atmosphere, so there is less recoil (remember, 1/2 mass X velocity squared!!)

I read your other thread with some interest, having just gone through a similar decision process with a 1893 Mauser. I wanted to choose a cartridge I could convert to that would not run the risk of some ijit overloading it at some future unknown time. I considered the 450 Marlin, 444 Marlin, 405 Winchester, and of course all the X57mm cartridges. Finally decided on 7.62X39, cheap to shoot and enough oomph to down a deer at modest ranges. It's all chambered and feeds, goes bang, ejects, etc. I finally figured out I need an elk thumper like I need a bloody nose.

[Saskhunter]

One thing that is missing in the bolt thrust calculations is the friction of the case on the chamber. I have no idea how much of that calculated thrust is "absorbed" by the fact that the case is stuck to the sides of the chamber quite hard during firing, but I would expect it to be a considerable proportion. The only reason you can eject a fired case afterward is the fact that the case springs back a bit after firing and releases itself. It takes a hammer to get a stuck case loose if an over loaded round is fired. That friction absorbs thrust.

[Bart B.]

Barnbwt, are those pressures in actual pounds per square inch (PSI) as measured by strain gauges, or are the copper units of pressure (CUP)?

I as 'cause far too often when CUP systems are used, the results are stated as PSI. There's quite a difference in actual pressure between them in the 50,000 CUP (60,000 PSI) range. Down in the 30,000 range for both, they're virtually identical. SAAMI specs for the .45-70 are 28,000 both CUP and PSI as shown in:

http://www.saami.org/specifications_...wnload/206.pdf

[Art Eatman]

A rule of thumb is that single-lug bolts not be loaded for chamber pressures above 40,000 psi. Modern bolts are commonly loaded to 55,000 psi and even more. Safety testing is generally 50% or more above this.

Measure the base area of the bolt lugs in square inches. That lets you calculate the shear force on the lugs in psi. Odds are it's beaucoup less than the yield strength of the steel.

[Brian Pfleuger]

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.

http://www.varmintal.com/a243z.htm

[barnbwt]

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

TCB

[Mobuck]

Don't be fooled or mislead by some country's war time expedient chambering practices. When things are desparate, you can take chances with SOMEONE ELSES's face, eyes, and fingers.
Some advice from a guy who lost a lot of blood and has over 12" of scars on his face-any rifle old enough to have the original chambering you noted should probably not be fired at all let alone be rebarrelled/rechambered. The rifle that almost killed me was a model reputed to be"one of the strongest" but had a slight flaw which ended in a catastrophic failure.

[Art Eatman]

Regrettable, Mobuck, but tens of millions of ancient rifles have performed with no problems due to faulty metallurgy.

Sure, rifles made late in a war by the losing side had problems. The Nazis began having sabotage problems in 1943, which increased in 1944--which provides cutoff dates.

The Japanese rifles' quality declined by sometime in late 1944, from what I've read.

[Gunplummer]

P.O. Ackley had a picture in his book of a Model 70 action blown apart due to faulty materials. Nothing is ever 100 % sure.

[JohnKSa]

...far too often when CUP systems are used, the results are stated as PSI. There's quite a difference in actual pressure between them in the 50,000 CUP (60,000 PSI) range. Down in the 30,000 range for both, they're virtually identical. SAAMI specs for the .45-70 are 28,000 both CUP and PSI...

While both CUP and PSI are used to specify allowable pressures, they measure those pressures very differently and it's not possible to make accurate general assessments how they will relate to each other beyond the very broad, and not very useful, statement that a as CUP figures increase, PSI figures will also generally increase and vice versa.

Dr. Oehler did an extensive analysis of this topic and his assessment was that "...there is no way to predict CUP from a peak PSI reading, and there is no way to predict a peak PSI reading from a CUP reading."

It's not a good idea to assume, in the general case, that CUP and PSI figures will be "virtually identical" based simply on the pressure range that the cartridge fits into, e.g. under 30KPSI.

For example, the .44Special has a CUP rating of 14,000 but a PSI rating of 15,900 and the .45ACP has a CUP rating of 18,000 with a PSI rating of 21,000 even though both cartridges are well under the 30KPSI limit.

http://www.saami.org/specifications_...wnload/205.pdf

[barnbwt]

LMobuck, it is precisely for that reason I wouldn't shoot one in8mm Mauser; higher pressures and slightly higher bolt thrust. I also won't shoot Nazi surplus as well. I'll be frequently checking the lugs for cracks or peening no matter what I do.

TCB

[Jim Watson]

Which I think reason enough to stick to SAAMI .45-70 (if it will fit the action).
Or is a 405 at 1700+ insufficient?

[Scorch]

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.

Absolutely true, so there is some room to argue. But rather than get angles and cosines of the chamber walls, derive the coefficient of friction, and test the brass for malleability, it is easier to approximate the bolt thrust by chamber pressure X case head area. It may not be precise, but then neither are a lot of other values we throw around. But this I can say fairly confidently: higher chamber pressure will translate into higher bolt thrust.

Anyway, since your 95 is designed for rimmed cartridges and you want an impactful chambering, I would take a serious look at 450 Marlin, 444, 405, 348, 30-40 Krag, in that order. 45-70 does not leave a lot of room at the edge of the bolt for stuff like extractors, bolt face relief, etc.

[barnbwt]

Which I think reason enough to stick to SAAMI .45-70 (if it will fit the action).
Or is a 405 at 1700+ insufficient?

Well, it's good to know what the limits of a conversion are, especially when loads are as variable/poorly defined as many of the "modern" .45-70s. I'd hate to rechamber a rifle that could hurt someone with an otherwise safe/common loading. It's comforting to know your 45-70 rifle can also safely shoot full-on 50AK . I'll most likely never shoot the thing anywhere near max simply due to the pain, unless there's a pad out there made of magic or something

At any rate, the reason I opened this thread was to see if I was approaching the bolt-load question from the proper scientific perspective, or if I was totally off base. I appears I'm on the right track; asking the right questions, anyway.

If there is a reliable way to calculate case-friction, someone let me know. I'll bet it's more important in the current chambering at 50,000psi than down around 30000psi, and I need to know if lowering the pressure could actually increase the bolt thrust .

Also, how much force can a cartridge carry before it simply blows out to fill the chamber (stretch or case head separation)?

*Unrelated question: have any of you Contender/Handirifle guys experimented with Sptizer (not rubber-tip FTX) bullets in the 45-70? Does it work, or is performance the same? Would weird barrel twist be needed? I ask because 45-70 is a good deal shorter than the original round, and is in a box magazine; so I'd have more options than the lever guys .
TCB

[Jim Watson]

I saw the calculations once but no longer have the numbers.

Look at a fired case, you can usually see the expansion ring where the brass gripped the chamber... until it didn't. Cross section there, measure and do the geometry to get the actual thickness of brass. Multiply that by the tensile strength and you will get the load necessary to stretch brass back into the headspace. Which I recall is not a lot compared to what is transferred to the locking lugs. The case isn't holding much, it is mostly just a sealing gasket.


The BPCR guys did some work with .45-XXX spitzer shapes a number of years ago. They concluded it did not help much and the present crop of "money bullets" has an ogive calculated to give the best results transsonic.
Of course you can launch them faster with smokeless.
There was an outfit making .458 spitzers but I no longer have the references to find them, if even still in operation.

[Axelwik]

Unrelated question: have any of you Contender/Handirifle guys experimented with Sptizer (not rubber-tip FTX) bullets in the 45-70? Does it work, or is performance the same? Would weird barrel twist be needed? I ask because 45-70 is a good deal shorter than the original round, and is in a box magazine; so I'd have more options than the lever guys .
TCB

There are some pointed bullets for the 45-70, cast lead as well as jacketed. Hornady has some with a soft rubber tip with higher BC that can be used in a tubular magazine. http://www.hornady.com/store/leverevolution

[2damnold4this]

8x56R, the "original" chambering

If I'm not mistaken, the original chambering for the M95 was the 8x50R.



I don't think it would be a problem to rechamber and shoot .45-70 as the rifle is certainly stronger than an old trapdoor.