Pressure wave visualization

[MarinePMI]

Hey all,

I was out this weekend, working up a load for a 6mm CM, using fairly light bullets and slow(ish) powder (H-4350) for that weight of bullet. Suffice it to say, groups were good at 100yds, but vertical dispersion (and ES's) were all over the place. As I ejected the cartridges, the cases came out in a variety of conditions, showing obturation of the case not occuring consistently. This was with virgin Lapua SRP brass, 46grs of 4350 (compressed load) under a Sierra 70 gr Blitzking bullet (3540fps). As I was thinking about it, I thought to myself this may make for a good opportunity to show (visually) what happens when a case is fired, and the pressure wave builds, causing the case to be pushed out against the walls of the chamber, thereby sealing it. In this case I just happened to be at the point where the case sometimes was not sealing as ignition began (low pressure spike). At any rate, in the picture you can see the one I annotated, where the gases escaped, and as pressure built, you can see it seal up the case in stages. At the lowest is the leaking gas, then the case began to seal at the shoulder/body junction, and then finally at the base of the case neck.

Anyways, I thought it was a good example of demonstrating the pressure wave, as it builds in the case, and moves forward, from initial ignition to a sufficient pressure to seal the case. Several other cases show various stages of pressure peak sealing the case.

 

[MarinePMI]

Definitely need to move to a faster burning powder...

 

[Muir]

H414. ~Muir

 

[MarinePMI]

H414. ~Muir

Gonna go the BLC-2 route or CFE223. Two separate reloading manuals list them as one of the higher (more accurate) loadings. And I've got a couple 8lb kegs of each. May even try Varget...

 

[Muir]

Gonna go the BLC-2 route or CFE223. Two separate reloading manuals list them as one of the higher (more accurate) loadings. And I've got a couple 8lb kegs of each. May even try Varget...

Sounds good. I'm just basing my experience with light bullets in the 243. H414 the best.~Muir

 

[Smellydog]

Hated 414 for 100gn. 70gn good.
@MarinePMI how much 4350 will the case hold if brimed?

 

[borbal]

Mmmm... Nah. That is not how it works.

One thing you definitely do not want in your gun is 'pressure waves'. Normally, it is a reasonable approximation that all the powder is ignited at the same time and the pressure behind the projectile is uniform. That is the 'classical' internal ballistics picture and how a gun works in normal operation.

However, the rate at which propellant burns is proportional to the pressure of the gasses and so if there are any variations in pressure, then pressure waves can set up which propagate back and forth between the breech and base of the projectile. As these waves propagate, the pressure in the wave builds up in a non-linear way and can lead to catastrophic destruction of the gun.

This is not really a problem in small-arms, but it definitely is a potential problem in large guns and most of the research in internal ballistics since the war has been dedicated to understanding pressure waves and how to mitigate them.

Anybody who has ever played with measuring pressures in rifles though will have seen large spikes in pressure occurring and they are much more common than you might think in relatively standard loads, though the results are very rarely catastrophic.

There is a known problem when using reduced loads of slow powder in magnum rifles which is why minimum loads are quoted. This seems to be caused by the powder being blown up against the back of the bullet by the primer, where it is compressed and then detonates. When propellant detonates, all the propellant turns to gas instantaneously and the result is invariably catastrophic. However, this is not the 'pressure wave' phenomenon described above.

 

[Smellydog]

Mmmm... Nah. That is not how it works.

One thing you definitely do not want in your gun is 'pressure waves'. Normally, it is a reasonable approximation that all the powder is ignited at the same time and the pressure behind the projectile is uniform. That is the 'classical' internal ballistics picture and how a gun works in normal operation.

However, the rate at which propellant burns is proportional to the pressure of the gasses and so if there are any variations in pressure, then pressure waves can set up which propagate back and forth between the breech and base of the projectile. As these waves propagate, the pressure in the wave builds up in a non-linear way and can lead to catastrophic destruction of the gun.

This is not really a problem in small-arms, but it definitely is a potential problem in large guns and most of the research in internal ballistics since the war has been dedicated to understanding pressure waves and how to mitigate them.

Anybody who has ever played with measuring pressures in rifles though will have seen large spikes in pressure occurring and they are much more common than you might think in relatively standard loads, though the results are very rarely catastrophic.

There is a known problem when using reduced loads of slow powder in magnum rifles which is why minimum loads are quoted. This seems to be caused by the powder being blown up against the back of the bullet by the primer, where it is compressed and then detonates. When propellant detonates, all the propellant turns to gas instantaneously and the result is invariably catastrophic. However, this is not the 'pressure wave' phenomenon described above.

Utter nonsense pure speculation / opinion.

 

[MarinePMI]

Hated 414 for 100gn. 70gn good.
@MarinePMI how much 4350 will the case hold if brimed?

Maybe 46.5 gr's. 46gr's comes up to the base of the neck if I use a drop tube and tap the case.

 

[MarinePMI]

Mmmm... Nah. That is not how it works.

One thing you definitely do not want in your gun is 'pressure waves'. Normally, it is a reasonable approximation that all the powder is ignited at the same time and the pressure behind the projectile is uniform. That is the 'classical' internal ballistics picture and how a gun works in normal operation.

However, the rate at which propellant burns is proportional to the pressure of the gasses and so if there are any variations in pressure, then pressure waves can set up which propagate back and forth between the breech and base of the projectile. As these waves propagate, the pressure in the wave builds up in a non-linear way and can lead to catastrophic destruction of the gun.

This is not really a problem in small-arms, but it definitely is a potential problem in large guns and most of the research in internal ballistics since the war has been dedicated to understanding pressure waves and how to mitigate them.

Anybody who has ever played with measuring pressures in rifles though will have seen large spikes in pressure occurring and they are much more common than you might think in relatively standard loads, though the results are very rarely catastrophic.

There is a known problem when using reduced loads of slow powder in magnum rifles which is why minimum loads are quoted. This seems to be caused by the powder being blown up against the back of the bullet by the primer, where it is compressed and then detonates. When propellant detonates, all the propellant turns to gas instantaneously and the result is invariably catastrophic. However, this is not the 'pressure wave' phenomenon described above.

Not so sure about detonation in rifles, as that has been debated ad nauseum with no clear proof one way or the other. But, that being said, your last part, about the powder being pushed against the base of the bullet, does have some merit. I believe it was Narramore that initially discussed this back in the late 1800's/early 1900's. He referred to it as "slugging the bore", where the unlit powder being pushed against the base of the bullet, didn't so much cause detonation, rather it was that the ignited powder was pushing up against a bullet (it's weight) as well as pushing against the weight of the unlit powder itself. Effectively creating a situation that was pressure trying to move too high a mass down the bore. And as a result, pressure spiked early, and sometimes catastrophically. Yes, all the powder did eventually ignite, but the premise was that it was the increase in mass that caused fatigue/failure of the rifle receiver/bolt, not so much things "detonating" from powder ignition itself.

But again, this is largely still theory, and everyone has their own.

 

[borbal]

Utter nonsense pure speculation / opinion.

Wot.... all of it?

 

[Andy RV]

What you’re seeing there is varying levels of obturation due to the decrease in wall thickness and hardness towards the mouth of the case, rather than a pressure wave.

Simply, the harder and thicker the material the more pressure is required to cause it to deform, elastically to plastic(to a degree) within the chamber which is producing the witness marks you can see.

 

[MarinePMI]

What you’re seeing there is varying levels of obturation due to the decrease in wall thickness and hardness towards the mouth of the case, rather than a pressure wave.

Simply, the harder and thicker the material the more pressure is required to cause it to deform, elastically to plastic(to a degree) within the chamber which is producing the witness marks you can see.

And is then not, the pressure wave traveling down the case wall, and out the neck? That's kind of my point...

 

[Smellydog]

Wot.... all of it?

where it is compressed and then detonates

Low density spherical powders need a level of compression. It is not compression that causes them to detonate but rather the lack of it.
Certain spherical powders are difficult to ignite ( relatively). If too loose, the flame front struggles to pass from one kermal to another.
It can result in , for want of a better description, flame out or a deminished flame front.
However a reaction has started, heat is involved, gasses are being produced.
The progressive properties of said powder are now lost or changed. Should reignition take place it can be a detonation.
Regards the mass of the powder adding to the load or work to be done it's all ready a factor. It's not going to add to a failure.
Crimping is a good move with slow spherical. Anything that helps the flame front migrate before to much expansion is a good thing.

There is an issue with igniting large weights of powder charges in artillery and many attempts have been made to lower breech pressure and heat generated to preserve barrels.
Elmer Keith I believe developed for 50bmg a flash tube case that ignited the powder just under the bullet first. It lowered pressure and heat but gave the same performance and also preserved barrels for longer.
I digress.....
Sorry for being a grump earlier

 

[Andy RV]

And is then not, the pressure wave traveling down the case wall, and out the neck? That's kind of my point...

Not quite, yes there will be an initial pressure wave as the burn progresses through the charge however this minor wave is soon overtaken by a the rapid pressure rise which is acting equally upon the entirety of the internal surface area of both the case and barrel. Peak pressure does not occur while the bullet is still within the case, it occurs within the first 1/4 (perhaps a little less) of the barrel length, hence the pressure is equally distributed throughout the volume of the system rather than a wave.

Personally I’d associate a pressure wave with the detonation of explosives, propellants (The vast majority of the time luckily!) deflagrate with a rapid but controlled pressure rise.

 

[webley701]

Not so sure about detonation in rifles, as that has been debated ad nauseum with no clear proof one way or the other. But, that being said, your last part, about the powder being pushed against the base of the bullet, does have some merit. I believe it was Narramore that initially discussed this back in the late 1800's/early 1900's. He referred to it as "slugging the bore", where the unlit powder being pushed against the base of the bullet, didn't so much cause detonation, rather it was that the ignited powder was pushing up against a bullet (it's weight) as well as pushing against the weight of the unlit powder itself. Effectively creating a situation that was pressure trying to move too high a mass down the bore. And as a result, pressure spiked early, and sometimes catastrophically. Yes, all the powder did eventually ignite, but the premise was that it was the increase in mass that caused fatigue/failure of the rifle receiver/bolt, not so much things "detonating" from powder ignition itself.

But again, this is largely still theory, and everyone has their own.

Earl Naramore started writing in the late 1930’s (he was a contemporary of Hatcher) but wrote mainly in the 1950’s to 60’s.

It’s not ‘theory‘ you were referring to, it’s ‘hypothesis’.

 

[MarinePMI]

Not quite, yes there will be an initial pressure wave as the burn progresses through the charge however this minor wave is soon overtaken by a the rapid pressure rise which is acting equally upon the entirety of the internal surface area of both the case and barrel. Peak pressure does not occur while the bullet is still within the case, it occurs within the first 1/4 (perhaps a little less) of the barrel length, hence the pressure is equally distributed throughout the volume of the system rather than a wave.

Personally I’d associate a pressure wave with the detonation of explosives, propellants (The vast majority of the time luckily!) deflagrate with a rapid but controlled pressure rise.

I'm beginning to think we each speak a different version of English...

 

[MarinePMI]

Earl Naramore started writing in the late 1930’s (he was a contemporary of Hatcher) but wrote mainly in the 1950’s to 60’s.

It’s not ‘theory‘ you were referring to, it’s ‘hypothesis’.

Then maybe I'm thinking of Pope...

As to theory versus hypothesis, I think everyone understood what I was saying, but thank you.

 

[Dickie]

I'm sure you yanks just come on here for the wind up.
@MarinePM1, @Muir