Unraveling the mystic of firearms design

[Hannibal]

Trying to hash some things out in my mind. Hoping someone here can help me out.

Is it true that the cartridge pressure is mainly confined to the barrel on a rifle, no matter if it's a single shot, block action, bolt action or repeater? There is bolt thrust I understand which I will inquire about in my next question.

Is the primary function of the receiver to contain the bolt thrust? Secondary function to facilitate cartridge installation and removal, third function to facilitate ignition?

I suppose a forth function would be stock mounting?

I realize these are oversimplification. I find the best way to understand complex problems is to break them down into small parts, the smaller the better.

Perhaps there are references available to aid my understanding?

Thanks for looking and entertaining my rambling.

[john.k]

Base thrust on a perfectly freely moving case is simply pressure X area........as the area is of the order of .25 sq in,then ,for instance ,base thrust with 20 tons/sq in is 5 tons .....this is a force easily resisted by a small hardware store bought fastener ....... a barrel is a more complex item ,as thin shell pressure calculations are not applicable,and its best to go with rule of thumb .......such as a barrel should be 2x the diameter of the hole inside .......so a 45/70 would be 1" dia at the chamber ............this is clearly not so with many lever actions ,such as the Marlin Model 95,which must surely be on the razor edge of blowing apart....or not.

[Hannibal]

Base thrust on a perfectly freely moving case is simply pressure X area........as the area is of the order of .25 sq in,then ,for instance ,base thrust with 20 tons/sq in is 5 tons .....this is a force easily resisted by a small hardware store bought fastener ....... a barrel is a more complex item ,as thin shell pressure calculations are not applicable,and its best to go with rule of thumb .......such as a barrel should be 2x the diameter of the hole inside .......so a 45/70 would be 1" dia at the chamber ............this is clearly not so with many lever actions ,such as the Marlin Model 95,which must surely be on the razor edge of blowing apart....or not.

Could this possibly be due to the pressure curve of black powder propellants back when the cartridge was designed?

Obviously some designs are capable of much higher pressures. I do like that you referenced the 45-70.

[Winger Ed.]

The receiver holds everything together, but has to be more than strong enough to hold the bolt thrust.
The pressure from the burning powder goes out equally in all directions--- if it can, and until it finds a way to escape such as out the barrel.

The more surface area the bolt it has (read that as more locking lugs) to spread out that pressure,
the more slim & trim and less 'bulky' the receiver has to be.
That's why you don't see single locking lug bolts like some ancient Mausers had on modern and more powerful weapons.

[Winger Ed.]

Could this possibly be due to the pressure curve of black powder propellants back when the cartridge was designed?

If accuracy is less of a concern than weight, BP barrels can get pretty thin.
Some old Infantry muskets used mostly for volley fire look like the barrel isn't much more than sheet metal.
But the long range BP cartridges for 'buffalo guns' had the heavy/thick barrels.
It was more about accuracy than concerns of pressure.

When a bullet goes down the barrel, it wants to whip it around and vibrate it like ringing a bell.
As it does that, the barrel moves, swells, bows up & down. All sort of actions are going on as the bullet goes down it.
Thicker/heavier barrels minimize this effect and are more accurate since the barrel isn't
whipping around (as much) when it turns loose of the bullet.
Hold a rifle barrel and fire it sometime. You'll think its going to flex, vibrate, and jump out of your hand.

After a safety point is reached:
Barrel thickness is more about the compromise between accuracy & weight than worrying about pressure.

Since they don't usually carry certain rifles all day while out hunting--
That's why the bench rest folks have those rather short-ish, and real heavy bull barrels like they do.

[Hannibal]

The receiver holds everything together, but has to be more than strong enough to hold the bolt thrust.
The pressure from the burning powder goes out equally in all directions--- if it can, and until it finds a way to escape such as out the barrel.

The more surface area the bolt it has (read that as more locking lugs) to spread out that pressure,
the more slim & trim and less 'bulky' the receiver has to be.
That's why you don't see single locking lug bolts like some ancient Mausers had on modern and more powerful weapons.

Ancient weapons didn't have pressures exceeding 60,000psi either. Add to that the 'idiot' factor who thinks if the powder will fit in the case then it's all good and the potential problem is apparent.

I've never really understood the desire to push the velocity and/or pressure. But I never wanted to hit a target at a mile, either. I guess I'm simple minded that way.

[Hannibal]

If accuracy is less of a concern than weight, BP barrels can get pretty thin.
Some old Infantry muskets used mostly for volley fire look like the barrel isn't much more than sheet metal.
But the long range BP cartridges for 'buffalo guns' had the heavy/thick barrels.
It was more about accuracy than concerns of pressure.

When a bullet goes down the barrel, it wants to whip it around and vibrate it like ringing a bell.
As it does that, the barrel moves, swells, bows up & down. All sort of actions are going on as the bullet goes down it.
Thicker/heavier barrels minimize this effect and are more accurate since the barrel isn't
whipping around (as much) when it turns loose of the bullet.
Hold a rifle barrel and fire it sometime. You'll think its going to flex, vibrate, and jump out of your hand.

After a safety point is reached:
Barrel thickness is more about the compromise between accuracy & weight than worrying about pressure.

Since they don't usually carry certain rifles all day while out hunting--
That's why the bench rest folks have those rather short-ish, and real heavy bull barrels like they do.

Now you've got me wondering just what pressure it would take to rupture an unobstructed barrel.

There is a number out there and it's going to be diameter dependant.

[Hick]

Base thrust on a perfectly freely moving case is simply pressure X area........as the area is of the order of .25 sq in,then ,for instance ,base thrust with 20 tons/sq in is 5 tons .....this is a force easily resisted by a small hardware store bought fastener ....... a barrel is a more complex item ,as thin shell pressure calculations are not applicable,and its best to go with rule of thumb .......such as a barrel should be 2x the diameter of the hole inside .......so a 45/70 would be 1" dia at the chamber ............this is clearly not so with many lever actions ,such as the Marlin Model 95,which must surely be on the razor edge of blowing apart....or not.

Yes, but... you are not mentioning fatigue cracking. The endurance limit of typical firearm steels is much lower than the ultimate strength. The endurance limit is the stress a steel can take in repeated load cycles without developing cracks. The surfaces that hold the bolt in place (bolt lugs on a bolt action) are themselves small in surface area and the bolt lugs have to have stresses below the endurance limit. This is also important. If the stress is too far above the endurance limit the firearm is limited on number of rounds that can be fired successfully.

In addition, firearms are proof tested at loads above the cartridge specifications in order to detect any hidden defects in the metal, so the barrel, receiver and bolt are sized to handle the proof test loads, which vary depending on design, but can be 50% or more above the maximum cartridge pressure

These are all factors in determining the sizing of the parts and that includes the receiver.

[Winger Ed.]

Ancient weapons didn't have pressures exceeding 60,000psi either.

Exactly. A weapon has to be designed with the materials, and for the cartridge it is intended for..... It's always been that way.

Be it a .58 cal. musket, a .22LR, a .30 cal. sniper rifle, Grandpa's .30-30, or a .50BMG- The same principles apply.

[john.k]

Fatigue cracking typically happens after millions of cycles ......Ive seen plenty of cracked parts in M60 MGs ,and no doubt cracking happens in other MGs too........Ackley demonstrated that bolt thrust with dry cases and dry chamber may well be zero......The highest back thrust will typically occurr when a case breaks just in front of the solid head ,or if its greased.

[Cap'n Morgan]

Interesting tidbit: Gunmaker of yore, W. W. Greener wrote in his book 'The Gun and it's Development' that the pressure in a gun would explode the barrel if it were of longer duration than the few milliseconds a shot normally lasts. I find this hard to believe, in fact I think it's pure baloney; pressure is pressure, and time has little to do with it.

[Winger Ed.]

Now you've got me wondering just what pressure it would take to rupture an unobstructed barrel.
There is a number out there and it's going to be diameter dependant.

That, and wall thickness, as well as how strong the steel is that it's made from.
I've not heard of an unobstructed barrel letting go.
Bolt locking lugs cracking/failing, the lower part of a bolt face blowing off on a M1A,
and blown cases that vented out like they're supposed to, but not the barrel itself.

[Wayne Smith]

I think it was PO Ackley who experimented with this. He cut the chamber of a Springfield rifle (30-06) and fired cartridges in it - normal cartridges, not proof. He kept cutting it until it let go when fired. I don't remember precisely the thickness, but it was close to the thickness of a piece of paper! He stated that the cartridge case handled most of the pressure.

[dtknowles]

Now you've got me wondering just what pressure it would take to rupture an unobstructed barrel.

There is a number out there and it's going to be diameter dependant.

Calculating the pressure that would burst a barrel is not all that complicated. What is complicated is what pressure the barrel sees as the pressure drops quite quickly as the bullet moves down the barrel.



Run the equation for a 45 cal barrel, 90 ksi ult steel and 40 ksi pressure I get a wall thickness of 0.366" for a barrel OD of 1.19. Remember this it burst with no margin. The barrel would bulge at a much lower pressures.

[dtknowles]

I think it was PO Ackley who experimented with this. He cut the chamber of a Springfield rifle (30-06) and fired cartridges in it - normal cartridges, not proof. He kept cutting it until it let go when fired. I don't remember precisely the thickness, but it was close to the thickness of a piece of paper! He stated that the cartridge case handled most of the pressure.

I think this is nonsense. Thin brass surrounded by thin steel is not going withstand the pressure of a .30-60 cartridge. In the calculation I provided in a previous post, it takes more than 3 tenths inches of mild of steel to withstand that chamber pressure at that diameter. A few hundredths of inch of brass is not going to help much.

Tim

[Recycled bullet]

Brass cartridge casings are a gas sealing gasket. It expands and contracts during the firing and cycling operation. When this works correctly the gun is safely fired and the casings are reloadable.

The gun's metal barrel support is required to support the gasket.

When the gun fires the primer it throws a incredible fire blast into the powder column and ignites it. The hot expanding gases apply incredible forces in all directions evenly inside the casing. If this pressure is high enough it fire forms. When the gas forces exceed the neck tension of the cartridge the bullet starts to move. Remember the hot expanding gases will flow to equilibrium.

This is why out of battery discharges are so dangerous. The gasket is unsupported and may burst. Loose or excessive gun headspace can be dangerous for the same reasons of lack of brass support or can lead to broken or battered bolt lock up lugs.

This is an interesting rabbit hole to fall into.

[john.k]

It was Springfield Arsenal that experimented with barrels as thin as 1/16".the experiments are described in Hatchers Note Book.

[JSnover]

It was Springfield Arsenal that experimented with barrels as thin as 1/16".the experiments are described in Hatchers Note Book.

Correct, and Hatcher opted to discontinue testing at roughly 1/16" barrel thickness. Not because the barrels burst - they didn't - but because he couldn't see much point in trying to turn them any thinner.
This supports the belief that it IS more about time than pressure.

[JSnover]

For what it's worth, working surface applies to the chamber and barrel thickness and diameter as well. As the pressure is higher at the chamber end, naturally we need thicker walls in that area. But in any pressurized tube or cylinder, as the inside diameter increases the working surface increases. The tube wall thickness must also increase to handle the greater total applied force.
The reason tapered barrels work is because the pressure curve drops along the length of the barrel during the shot. The reason straight barrels work is because they're already thick enough to handle the pressure spike, it's a downhill ride from there.

[dtknowles]

It was Springfield Arsenal that experimented with barrels as thin as 1/16".the experiments are described in Hatchers Note Book.

What cartridge and barrel contour? Were they only 0.0625" thick in the chamber section? Did they use normal barrel steel or something stronger?

A 30-06 barrel would not burst even if only 0.0625" thick at the muzzle but closer to the chamber it would. I am sure Hatcher knew and understood that before he even started the testing.

The idea that the brass cartridge case is going to be able to hold the pressure without the surrounding ring of steel is untrue. Even at semiauto pistol pressures unsupported brass in thick head section of cartridge cases has low margins against failure and often bulges.
If you fire a .32 Colt round in a .32 S&W chamber the brass splits and .32 Colt is as low a pressure cartridge as you are going to find.

Another consideration is that steel yields before it fails so pressures that do not burst a barrel can still ruin a barrel as the yielded barrel will now have a bulged section where the bore is larger.

Tim