Chamber Pressure in Regards to Cartridge Selection


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barnbwt
November 10, 2012, 11:43 PM
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 ever 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 (http://en.wikipedia.org/wiki/.50_Alaskan)
From Wiki (not the best source for load data :o)
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 (http://en.wikipedia.org/wiki/8x56r)
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) (http://en.wikipedia.org/wiki/8mm_mauser)
Pressure: 57,000psi
Base Diameter: .470in
-Area: .174in^2
Calculated bolt thrust: 9887.3lb (hope those lugs had a +20lb safety factor:uhoh:)

45-70(the round I plan to rechamber to) (http://en.wikipedia.org/wiki/45-70)
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:D)

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?:confused: 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

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Jim K
November 12, 2012, 03:09 PM
No, you are correct to a point. The rearward thrust is based on the area of the inside rear of the case with a bit of trig to take the curve of the base into consideration. IIRC, the inside area of the .30-'06 size case is roughly 0.1 square inches, so for a 50,000 psi round the backthrust would be about 5000 pounds (absolute, not per square inch).

But the locking lugs are not really the weak link in a rifle, the case is. The case is made (usually) of brass, and under extremely high pressure will melt like a warm chocolate bar and extruded itself into the bolt face. If the case head is not supported fully (think '03 Springfield or the old style Winchester 70, the case will let go and wreck the action no matter how strong the locking lugs are.

Recoil is related only indirectly to pressure. As you say, it is momentum, the reaction to the action of the bullet movement. The formula is
Mass (b) times velocity (b) equals mass (r) times velocity (r) where b is bullet and r is rifle. Recoil begins at the instant the bullet begins to move, but the inertia of the heavier rifle means it will not show much of an effect until the bullet has left the barrel. (The effect is more rapid in a handgun and the sights take recoil into consideration.)

Jim

Clark
November 13, 2012, 11:50 AM
I have tried to blow up a lot of guns and I have calculated the max pressure of a lot of guns.
It neve matches.
It is always harder to blow up guns than predicted.

I would buy a couple extra rifles and try to blow them up. Steyr Mannlicher M95 rifles are practically free at gun shows. Less than $100 each. One's time blowing them up will cost much more.
When I started this hobby 12 years ago, I used a trigger string.
It was interesting to find out that my father was using a trigger string 70 years ago.

I have been behind a concrete wall pulling the string. My father was behind a tree. I have also put the gun underground and pulled the string. I only did that once. The explosion goes out and then in, and when it goes in, it sucks dirt into every part of the gun.

barnbwt
November 13, 2012, 06:07 PM
Did some more researchin'; I feel better about the "theoretical" strength of the conversion as I calculated it. Again, I don't intend to exceed the factory standard operating loads, even with the unknown remaining safety factor.

The projected (along bore axis) area of the inside of the cartridge gives the most accurate load values (makes sense, that's where the pressure is. Thanks, Jim K), but it is more common to get the value using the base outside diameter, to bake in a bit more safety margin (probably around 10% in the case of 45-70).

Also, according to Wiki (which actually has a pretty good article on this obscure subject), NATO uses lubricated cartridges to proof test guns to eliminate friction effects on the test data. Any grit, oil, mud, blood, or beer on the case would greatly affect the effect case friction has on bolt-thrust, so avoiding it entirely seems a wise testing move. Seeing as designers have been incorporating the loads from lubed cases for a while now, perhaps the conventional wisdom about oiled cases being dangerous needs revision, especially since case life is improved by the practice.

Though these two assumptions add to the bolt thrust value, they are useful for creating a safety factor that can account for natural variances in cartridge pressure. Wiki mentions extra-high cook-off pressures as an example.

I have tried to blow up a lot of guns and I have calculated the max pressure of a lot of guns.
It neve matches.
It is always harder to blow up guns than predicted.


Thanks for weighing in Clark, you're the go-to guy for destructive testing :cool:. Guns certaintly have a healthy (excessive) safety factor baked in, but since I don't know enough about the alloy of these guys I can't make a good estimate as to what my limits are based on metal. By limbo-ing under the original estimated bolt thrust, I guarantee a successful outcome. I'll definitely be buying some kite string to proof test it with no matter what it gets bored out for ;)

I'm really tempted to try it in 50 Alaskan, just because the prospect seems utterly ridiculous to me (like the 45-70 BFR) but is mathematically feasible. I'd be knocking on that unkown safety margin you're talking about, though, if I put very hot loads together (extrapolation is always a bad thing in engineering ;)). If the 45-70 works out, I'll try another in "fitty" so I can have one to shoot, and a ringer for the new guy at the range :evil:. It's hard to tell how hard these cartridges really hit, because just like with Mosin carbines, people greatly over- and under-exaggerate their experiences. With a good pad and stock, 50AK may be manageable for all I know :confused:

Looking at the numbers, the rumors about the 8mm Mauser conversions probably being a bad idea seem well-founded, since the bolt thrust is a bit higher than even the 8x56R rechambering. That said, I've not heard of increased failure rates aside from the crummy extractors on those. Very little data, though; way too little to stake one's eyes/face on.

TCB

Jim Watson
November 13, 2012, 06:13 PM
I have tried to blow up a lot of guns and I have calculated the max pressure of a lot of guns.
It neve matches.
It is always harder to blow up guns than predicted.

What about the durability of a gun shot regularly with loads at or above its design limits?

If you don't blow it up with an overload, do you wear it out fast?

Clark
November 13, 2012, 07:30 PM
I asked that of JonA, a hunter who also does stress anal for Boeing.
The graph he drew is still on his site, ~ 8 years later.
I was interested in the Rem700 bolt lugs at the time.

http://www.jonaadland.com/SN.jpg

Steel will not fatigue below some stress, but Aluminum will. You can see this is steel.

The way I read it is that if the SAAMI proof pressure are for 1.3 to 1.4 times max average pressure for ammo, then fatigue is precluded over the next 1,000 shots.

I had an ME prof in engineering ethics [35 years ago] that said that if a rocket blows up on the launch pad, we don't learn much.

I say if we blow up a gun with the first shot, we don't learn much. I like a work up that is stopped when some tiny damage is caused to the gun or brass. Then I have learned a threshold.

I don't don't advocate anyone else should do this. I am master of speed chess and jumping from one branch to another in a tree. I can also look at a gun and imagine which way the parts will fly.

Glen, the smartest guy I ever talked to on the phone at Sarco, died 10 years ago when his Lee Navy bolt came back at him. I have looked at those rifles, and the bolt reassembly is tricky.

brickeyee
November 14, 2012, 04:06 PM
Making things fail is of limited utility, unless you make a lot of them fail to average out piece to piece variations or are looking for margin numbers above some operating point.

Even then, knowing the conditions that something can be safely used is usually more important and useful.

i test semiconductors for use in space at cyclotons.

I can break any part i want.

Set it to maximum operating voltage, get it up near maximum operating temperature, hit it with particles that only arrive every few thousand years per square centimeter and it WILL fail (particular technologies like CMOS vs. BJT have different 'worst case' conditions).

It is usually not a useful exercise.

Knowing the part will survive for X years on orbit with some very high probability is a more useful result.

barnbwt
November 14, 2012, 07:33 PM
Yup, the good 'ol Fatigue Failure curve for crack propagation. Remember that from my materials course a few years back. IIRC, there is a rule of thumb for the fatigue limit based upon either Yield strength, or Modulus...I don't remember anymore. It's a moot point for my rifle, since I don't know its composition or temper, but is important for new designs or wildcats.

I say if we blow up a gun with the first shot, we don't learn much. I like a work up that is stopped when some tiny damage is caused to the gun or brass.
Yup, designs are basically unreliable after yielding sets in (especially repetitive cylic loads) for safety-critical applications in my book. Like case-friction; it's nice to know it's there, but not a good idea to peg your allowables to it.

I finally made contact with some folks who made successful 45-70 conversions at Weapons Guild (cool forum, way more supportive of stupid/crazy/bad ideas than the pragmatic fuddy-duddies here ;) j/k, you know I love you all *awwww*

45-70 will be the caliber I rechamber to. More thump than I'd ever want, more power than I need, a more common caliber, it fits, and won't come close to current stress levels. Perfect. Saves me a good deal on the barrel and molds, dies, etc. too. If reworking the mag isn't super expensive, I think it's possible to keep the price under 500$ +/- the cost of the rifle (100$):cool:

Now I just need to look into what rebarrelling entails so I can track down a good smith. Apparently these barrels screw in much like Mausers, so hopefully that won't be too hard.

IF this turns out successfull and IF it's still somehow now enough; it's good to know the 50 is out there...

TCB

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