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Action pressure and case expansion?

C

CalG

Diamond
Joined
Dec 30, 2008
Location
Vt USA
i've been enjoying shooting an early Chinese SKS. (Ammo is cheap but accuracy is disappointing)

One of the frequent topics regarding the fire arm is that of "blown primers", A problem that a peruse of the internet suggests is NOT confined to SKS. ;-)

To the topic.

Reading on the subject of chamber pressures as related to action forces, I see no inclusion of the casings "grip" on the chamber walls during the firing cycle.

Could someone enlighten me on how any "formula' could be put forth for the action loading with such an "unknown" variable such as "case friction"? Or is it a simple matter of area X pressure X coefficient of friction between brass(or steel) and steel.

NB This IS NOT a question about pressure on the primer !
 
You don't see the inclusion of the brass gripping the chamber because it is a meaningless inclusion.

Many engineers have calculated what happens when you polish the chamber super good and may experimentalists have measured the pressure as the progressively get shinier and shinier chambers. No effect on anything is the short answer.

Seems counterintuitive. Before I looked into it, I would have assumed that a super well polished chamber would allow the brass to slide out easier under peak pressure.
 
GunBum said:
You don't see the inclusion of the brass gripping the chamber because it is a meaningless inclusion.

Many engineers have calculated what happens when you polish the chamber super good and may experimentalists have measured the pressure as the progressively get shinier and shinier chambers. No effect on anything is the short answer.

Seems counterintuitive. Before I looked into it, I would have assumed that a super well polished chamber would allow the brass to slide out easier under peak pressure.
Click to expand...

IIRC it was Spain's CETME who actually fluted some of their assault rifle chambers for a bit of extra delay in a blowback action so they could handle a more powerful round without the bolt having to weigh a ton.

"Obturation" is one term worth research. Fatigue from X many cycles of it another.

"Type K" copper plumbing can typically survive and recover from at least seven hard-freeze cycles f'rinstance. "L" or "M" not. Similar physics.

And then.. even were a chamber so precisely fitted the round had to be chambered with a hydraulic press - eg: NO obturation or movement - the casing is subject to work-hardening even from the hammer-blow of the expanding gases. It wants to flow lengthwise if it has nowhere else to go. Enter case neck trimmers.

Those gases are also corrosive, even when not mercurous primed, Brass being rather susceptible to lots of chemical antagonists and their by-products.

All among the rich tapestry of case-life issues w/r reloading, but not so directly related to the shedding of primers, per se..

That's more likely to be simple lack of proper support from the face of the closed bolt in your particular SKS.

They did not even try to compete with Savage Arms as to consistency and precision, after all.

Bill
 
Bill

Though I like your poetic ramblings, the off topic deviation gets annoying.

Gun Bum
it is not hard to rationalize that as the surface finish goes "up", the actual contact are increases. No surprise that there would be little change based on chamber finish alone. the coefficient of friction between surfaces is the "figure of merit". The comparison of chamber pressures to measured bolt compressive forces would be of great interest.

Absolutes vs Relative , I suppose.
 
CalG said:
Bill

Though I like your poetic ramblings, the off topic deviation gets annoying.
Click to expand...

Haven't shed even one primer since 1950, nor even had one volunteer to rise-up. Hopefully it is off-topic in a useful direction.

Might want to check your SKS more closely for fit-up before chasing case theory. Primer faulting can be akin to the Canary in a coal mine. Pay attention to it.

Bill
 
Monarchist said:
Haven't shed even one primer since 1950, nor even had one volunteer to rise-up. Hopefully it is off-topic in a useful direction.

Might want to check your SKS more closely for fit-up before chasing case theory. Primer faulting can be akin to the Canary in a coal mine. Pay attention to it.


Bill
Click to expand...
Bill

As mentioned in the OP, this is not about primers, but that question did lead to the subject .
The rifle I have has not displayed any sign of any weakness or malfunction (except the 6 inch groups at 100 ;-0
 
CalG said:
Bill

As mentioned in the OP, this is not about primers, but that question did lead to the subject .
The rifle I have has not displayed any sign of any weakness or malfunction (except the 6 inch groups at 100 ;-0
Click to expand...

That ain't too shabby considering who built them, when, what simple goals were involved, and what strictures their economy imposed on getting to even those minimalist goals.

:)
 
There are too many variables to be able to predict how much chamber pressure the cartridge case actually holds. For sure the cartridge case can and does hold some back thrust to relieve some of pressure on the bolt lugs.
To my mind, it is a welcome extra safety factor but not something I would hope to quantify or rely on in any way.
 
https://www.shootersforum.com/wildc...bsite-about-ackley-improved-cartridges-2.html

Post #26 describes a series of interesting experiments exploring the effects of casing to chamber friction. No formulae are developed, but the general conclusions are interesting.

After reading more of that thread, I remembered why I saved it. The 30-30 AI is one of my favorite contender barrels, although my current go to deer rifle is a 35 Remington Rimmed. Brass is made from 303 British.
 
Arent cheap 7.62x39 cases steel.?Neverthelees this caliber in a quality bolt rifle from CZ or Zastava also gives poor accuracy,its designed for spraying bullets,quantity over quality.Regards John.
 
gnadoc said:
https://www.shootersforum.com/wildc...bsite-about-ackley-improved-cartridges-2.html

Post #26 describes a series of interesting experiments exploring the effects of casing to chamber friction. No formulae are developed, but the general conclusions are interesting.

After reading more of that thread, I remembered why I saved it. The 30-30 AI is one of my favorite contender barrels, although my current go to deer rifle is a 35 Remington Rimmed. Brass is made from 303 British.
Click to expand...

The way I read that implies that the head is only supported by the case walls which grip the chamber wall. That means the thin case wall is supporting chamber pressure times the head area in tension. For a typical case and pressure, that comes out something like 5-7000 pounds. That is expecting a lot from a little bit of brass.

Re the question of recoil from straight or tapered cases, the bolt, case, and barrel are all locked together and recoil velocity is the ratio between the stuff that comes out the barrel and the gun's weight. The mechanism and the way the force is transmitted to the gun's mass has nothing to do with it.

Bill
 
john.k said:
Arent cheap 7.62x39 cases steel.?Neverthelees this caliber in a quality bolt rifle from CZ or Zastava also gives poor accuracy,its designed for spraying bullets,quantity over quality.Regards John.
Click to expand...


John

If you can reference GOOD accuracy using any combination of commercial or reloaded ammo in any 7.62X39 chambered rifle, I'm all ears. I hear about the "terrible quality of cheap ammo", but my own efforts trying the more dear rounds have not produced ANY IMPROVEMENT. I'm all ears!
 
CalG said:
John

If you can reference GOOD accuracy using any combination of commercial or reloaded ammo in any 7.62X39 chambered rifle, I'm all ears. I hear about the "terrible quality of cheap ammo", but my own efforts trying the more dear rounds have not produced ANY IMPROVEMENT. I'm all ears!
Click to expand...

Valmet. The weapon, not the ammunition. And still, only "maybe". Be sitting down when you price one.

Otherwise? Going against the grain, mate. BIGTIME. No percentage in it.

Wadda yah think keeps an AK or SKS reliable in the hands of a coolie or goat-herder with max seventh-grade education IF he wasn't a drop-out to keep the family fed? Or never started?

And on a weapon that MUST function in the Gobi Desert, high Himalayas, or gooey rice-paddy mud - frozen of a Korean winter - with near-zero maintenance?

And so damned cheap to make it matters not if they get blowed up by the hundreds in artillery or air-strikes, or dropped in large lots on the field of battle by folk who will no longer draw ammo or rations that evening? Then recovered and re-issued.

Hint: Third-world 'battle rifle' sure as Hell dasn't involve a lot of costly s**t nor close fits, anywhere.

May as well try to teach a pig how to whistle.
Wastes yer time. Annoys the pig.

Go and find a used Savage or Remington turnbolt. Most any common *US market* caliber will be accurate right out of the box. Very.

Just don't take one to a barefoot war in muddy South East Asia or the sand and dust of the Middle East, is all. Likely to die of exhaustion trying to keep the b***h clean enough to function at all.


Bill
 
Monarchist said:
Valmet. The weapon, not the ammunition. And still, only "maybe". Be sitting down when you price one.

Otherwise? Going against the grain, mate. BIGTIME. No percentage in it.

Wadda yah think keeps an AK or SKS reliable in the hands of a coolie or goat-herder with max seventh-grade education IF he wasn't a drop-out to keep the family fed? Or never started?

And on a weapon that MUST function in the Gobi Desert, high Himalayas, or gooey rice-paddy mud - frozen of a Korean winter - with near-zero maintenance?

And so damned cheap to make it matters not if they get blowed up by the hundreds in artillery or air-strikes, or dropped in large lots on the field of battle by folk who will no longer draw ammo or rations that evening? Then recovered and re-issued.

Hint: Third-world 'battle rifle' sure as Hell dasn't involve a lot of costly s**t nor close fits, anywhere.

May as well try to teach a pig how to whistle.
Wastes yer time. Annoys the pig.

Go and find a used Savage or Remington turnbolt. Most any common *US market* caliber will be accurate right out of the box. Very.

Just don't take one to a barefoot war in muddy South East Asia or the sand and dust of the Middle East, is all. Likely to die of exhaustion trying to keep the b***h clean enough to function at all.


Bill
Click to expand...


Do you have any idea of how many insurgents caught a bullet from one of those Remington M 24's that shouldn't be used in sandy conditions?
 
paracongo308 said:
Do you have any idea of how many insurgents caught a bullet from one of those Remington M 24's that shouldn't be used in sandy conditions?
Click to expand...

Roughly, yes. Demanding work, and well done.

Also not even a statistical rounding-error to what has been dealt out of the cruder barrels of many millions of Shpagins, AK's & SKSen, 1940's Eastern front, Korea, 'nam ..or just the last year or so - Africa and Middle East.

Not defending 'em. Relied on a 12-foot berm and 60 Mike Foxtrot thousand laboriously laterite-filled and peneprime treated Osnaburg jute bandsags for that.
.
 
Butch Lambert said:
The kinda new 100 yd. BR small group record was shot with a 7.62X39 improved wildcat. Mike Stinnett shot a 5 shot .0077 group in 2013 I believe.
Click to expand...

Not arguing the round, and more'n one wildcat was inspired by tons of cheap brass more than other factors.

Ain't even gonna ASK if he did that in a salvaged SKS action, though..

:)
 
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