Pressure inside a bullet case

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I'm trying to understand som basic rifle bullet physics regarding pressure inside the gun barrel. For no particular reason I'm comparing the .308 and 30-06 cartridge. When reading specs on the two I get confused. They both have maximum pressure levels around 415 MPa, but how can that be when the 30-06 case is more than a centimeter longer than the 308? If they are both filled with propelant, won't the 30-06 produce higher pressure just from having a longer case? And if they both produce the same pressure - does that mean that bullets with identical mass go the same speed and have the same energy?
 

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  • #2
A.T.
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I'm trying to understand som basic rifle bullet physics regarding pressure inside the gun barrel. For no particular reason I'm comparing the .308 and 30-06 cartridge. When reading specs on the two I get confused. They both have maximum pressure levels around 415 MPa, but how can that be when the 30-06 case is more than a centimeter longer than the 308? If they are both filled with propelant, won't the 30-06 produce higher pressure just from having a longer case?
It's not just about peak pressure, but how long it acts.

And if they both produce the same pressure - does that mean that bullets with identical mass go the same speed and have the same energy?
Given the same caliber, the peak force and thus the peak acceleration will be similar. But the muzzle velocity will be different if higher forces act longer.
 
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It's not just about peak pressure, but how long it acts.
Are you thinking of barrel length?
 
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jim hardy
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If they are both filled with propelant,

Read your reloading manual. You don't "fill" the case with powder you put in a carefully measured amount of it. And the amount depends on the powder type.

Completely filling a cartridge case is begging to blow the gun apart and likely your face with it.

upload_2017-5-20_10-40-32.png

A common and dangerous mistake for beginners is to put two measures of powder instead of one into a cartridge
Modern guns are designed to handle accidental overcharge within reason
this WW2 era Mauser vents the hot gas out the side to aim it away from your eyes and hopefully let the action stay together.

upload_2017-5-20_10-38-14.png


If you're reloading, connect with an old timer, please.
 
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What a great photo of that destroyed receiver, Jim. That truly makes the point in spades!!

I am surprised in that I expected the receiver to be thicker at the location of the locking lugs.
 
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jim hardy
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Thanks guys for the kind words !

I'm no gunsmith
but i believe the locking lugs transfer lateral force from the bolt to keep the high pressure gas inside the sealed barrel chamber, the soft brass cartridge serving as a self energizing seal as it deforms against the chamber walls making a perfect seal against pressure. Bolt holds rear surface of cartridge against that internal pressure.
That keeps the high pressure gas surrounded by plenty of metal. Note his 415megapascals is about 60,000 PSI twice the yield of everyday steel.

First symptom of overcharging is split cartridge cases. If enough gas leaks out through a split case to pressurize that region behind the chamber surrounding the bolt face, imagine what is the hoop stress in that comparatively thin shell. That's why the vent out one side.

Mausers 1898 and newer have a second set of lugs oops just a third lug-jh at rear of bolt. Should an action blow apart at front as in the picture, it'd set those lugs free. Rear lug should keep the bolt from going backward through the soldier .

old jim
 
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Spinnor
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Do different powders have different burn rates? If so, more of a "slower" burning powder could be used while keeping the peak pressures fixed but giving a longer, larger overall impulse?

Edit, I'm worried burn is not the right word to use here?
 
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jim hardy
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Do different powders have different burn rates? If so, more of a "slower" burning powder could be used while keeping the peak pressures fixed but giving a longer, larger overall impulse?

Most assuredly they do have different burn rates.
Try a search on powder burn rate..

Never use secondhand powder. A reloader friend of mine blew up a gun . He'd bought some secondhand powder.
He sent his gun and his powder to Winchester at their request. They found the powder in his bottle was a mix of two different powders - one per the label and the second way more potent. Somebody had mixed two different bottles of powder together .
Luckily he wasn't hurt.

Read your reloading manual.

old jim
 

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