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Old April 19, 2013, 10:37 PM   #1
johnwilliamson062
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Receiver pressure limits

There is something I don't quite get. As I understand it the following is how MOST rifles work:
The bolt moves into battery and as it does lugs on the bolt connect with lugs on the barrel. The two are very tightly joined. The trigger is pulled and the barrel bolt form a closed system until the bullet leaves the barrel(well, close enough). At this point the bolt and barrel, still locked together recoil into the receiver, which recoils into the stock, which recoils into the shoulder. Why is it the bolt and barrel do not hold the pressure?

Why is it I can't but a 223 bolt, 223 barrel, and put it into any receiver that can handle equal recoil?
Why is it an AR upper can be made of polymer but a steel 1895 Steyr can't handle the pressure?
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Old April 20, 2013, 05:36 AM   #2
Gunplummer
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You can put an AR barrel and bolt into any receiver that you can make it fit in. The bolt locks into the barrel, not the receiver on an AR. You can make a wooden receiver for the two parts if you want to.
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Old April 20, 2013, 08:25 AM   #3
Art Eatman
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Yup. Radial force is contained by the walls of the chamber. Rearward force is contained by the locking lugs and their slots in the forward portion of the receiver.

The bolt handle of a bolt-action rifle, when it serves as a third lug, is a "case of in-case" of failure up front--which is an extremely rare event.

The pressures from the rapid combustion are indeed contained within the chamber and barrel.

Recoil is the normal reaction from the mass of powder and bullet going in one direction and the gun itself pushed back in the opposite direction.
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Old April 20, 2013, 09:36 AM   #4
PetahW
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Quote:
Originally Posted by johnwilliamson062

Why is it I can't but a 223 bolt, 223 barrel, and put it into any receiver that can handle equal recoil? -

[See below]


Why is it an AR upper can be made of polymer but a steel 1895 Steyr can't handle the pressure? -

At least four reasons:

1) AR bolts lock into the barrel / most bolt-action rifle's bolts do not - they lock into receiver abutments.

2) The steels of about 120 years ago (1895) is in no way the equal of today's modern steel alloys.

3) Although some older rifle's receivers were hardened, not all were, and some that were supposed to be, were done hap-hazzardly.

4) There is NO practical way to find out what undue stress may have been placed on an older BA rifle in the past 100-odd years, that may cause unseen issues with the metal and/or bolt fitment.

The first time an old rifle's metal fails could very well be the NEXT time YOU fire it.



I hope that helps.

.
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Old April 20, 2013, 09:49 AM   #5
drcook
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Also rearward force is lessened by molecular adhesion of the case wall to the chamber wall. That is why you can get a case head separation if there is any slop in the mechanism / bolt lockup. The case will stretch anywhere there is not adequate adhesion to stop it.

I shoot straight walled cases in my BPCR rifles and had a rough chamber (I polished it out). You could see the radial expansion due to the marks on the case and there was difficulty extracting the cases, as the brass tried to "stick" to the chamber.

Also in BPCR rifles if there is any moisture in the chamber (people use "blow tubes" to keep the fouling soft) and the case cannot adhere to the chamber walls, sometimes the bullet will grab the case and rip the front part of the case off.

High pressure always tries to flow to an area of lower pressure in the most direct route. The most direct route is out the sides of the chamber. So everything expands. If the barrel (or barrel extension) and receiver are strong enough to contain the pressure, the next shortest path is back through the receiver. If that is strong enough then the last remaining path to drop the pressure differential is shoving the bullet out the front of the barrel.

A "rung" chamber is simply a prelude to a failure that didn't happen because the pressure was released in time.

Also it is not "recoil" that is locking the mechanisms together. It is pressure. "Recoil" is simply the equal and opposite reaction to the projectile (that is entire projectile mass, bullet, gas, unburnt powder, in BPCR rifles, the under bullet wads, shotshell cups, etc).

Remember in school we were taught about every action having an equal and opposite reaction ? Recoil is the opposite reaction of what exits the end of the barrel.

Last edited by drcook; April 20, 2013 at 09:56 AM.
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Old April 20, 2013, 12:54 PM   #6
Slamfire
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PetahW: You created a really excellent and succinct reply. I am going to have to steal it in the future. Or parts of it.
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Old April 21, 2013, 12:09 PM   #7
johnwilliamson062
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Quote:
AR bolts lock into the barrel / most bolt-action rifle's bolts do not - they lock into receiver abutments.
This si the part i was missing. THe bolt action I have studied the most is my K31 and I believe it also locks into the barrel, which is probably why it is so expensive to re-barrel.
I will have to look at my marlin 22(the only other bolt gun I currently own) and see how it is set-up.

People have told me that if I want a modern straight-pull rifle I should just get an AR and plug the gas plug. This has made me realize making a dedicated rifle off of parts might not be too difficult.
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