Secondary Pressures

[Carcassonne]

I found this article interesting.

https://www.shootingsoftware.com/barrel.htm

I never heard of "secondary pressures."

It appears to only happen with very long barrels.



.

['ol shooter]

Interesting article. I was amazed the first time I saw a slo-mo of an AR type rifle barrel whipping.

[fguffey]

I found this article interesting.

https://www.shootingsoftware.com/barrel.htm

I never heard of "secondary pressures."

[rsrocket1]

I ran a number of Quickload sets with the same bullet, powder, barrel length and it looks fishy to me. The initial peak should be around 0.3 ms (the web page shows around 0.4 ms) and the bullet exits the muzzle at about 0.8 to 0.9 ms.



From what I can tell, the "secondary pressure spike" is occurring right at or after the time the bullet exits the muzzle.


This measurement system is composed of a strain gauge glued to the outside of the barrel and not a piezo pressure transducer or copper crusher that is drilled into the chamber.

http://archives.sensorsmag.com/articles/0599/0599_p93/

That means the sensor is based on stress exerted on the outside of barrel and can be subject to other external vibrations such as maybe a surface wave on the barrel skin (just a wild a-- guess). I seriously doubt that the entire inside of the barrel is being subjected to a 120,000 psi blast. It would also have to be some sort of supersonic detonation too because it would need to know that the bullet is where it's at (the timing says it's about 20 inches down the pipe) and the shock would have to travel back to the strain gauge glued to the outside of the chamber.

Since it ain't my gun and not my problem , I can easily suggest someone drill, tap and mount a true piezo transducer to the chamber and shoot the same load. I'll bet the transducer will not see the "secondary pressure spike" inside the chamber.

[fguffey]

I seriously doubt that the entire inside of the barrel is being subjected to a 120,000 psi blast.

A shooter near Houston TX. used a laser to align the scope with the barrel; problem, he forgot to remove the laser tool from the muzzle. He chambered a round and pulled the trigger' and that was the end of his barrel, it split. The barrel let go at the weakest point. Back in the days A-Square started out to destroy a M1917 rifle, just before they gave up they filled a 30/06 case with pistol powder and let it go. The rifle did not give up easily but finally the barrel let go. They hung the rifle on the wall and called it "The Buck Horn Rifle". There is a bullet maker in Alaska that has an identical barrel hanging on the wall.

F. Guffey

[Metal God]

The link is interesting for sure . I remember debating the secondary spike a couple years ago . Likely at the same forum and thread Guffey spoke of . At the time and what the link above seemed to suggest was that the secondary spike was localized to a specific spot in the bore . They also suggested a guy was able to predictably blow up a barrel at a specific spot using the secondary pressure spike .

I believe this is what was being referenced

. Public debate over whether these secondary spikes are real was finally put to bed when Charley Sisk at Sisk Rifles blew the end off two barrels. We have also verified changes to the rate of acceleration just prior to, and after these events. Case Closed, it is real!

I'll start by saying this is above my pay grade . I how ever don't understand how you can have a localized pressure event in a long cylinder . Right or wrong I think of it as an air compressor filling it's tank . Lets say you have a 10" diameter 3' long tank being filled with air . Now lets say the valve letting air in to fill the tank is at one end of the tank . As the tank fills does the pressure build equally throughout the tank or is the pressure higher at the end with the valve ?

The localized secondary spike does not seem possible do to the fact that as pressure tries to build in one specific spot in the bore . That pressure is dispersed/relieved do to all the empty less pressurized space around it .

The other theory of how you get that localized pressure spike was touched on again in that link

If there is insufficient gas produced by the powder (burn rate too slow), pressure behind the bullet will drop excessively. Then, as the bullet's rate of acceleration falls due to bore friction, gases may "catch up" to the bullet before it exits the barrel and produce a secondary pressure event. In the above load we believe the heat generated from initial ignition coupled with a secondary pressure event increased the burn rate of residual ball powder to near detonation.

Which again is interesting because you can almost see that happening . The thing is why wouldn't the pressure just back fill the bore behind the bullet rather then some how create a localized secondary spike ?

It may happen , I'm just having a hard time understanding it and to be clear I'm not debating if there is or can be a secondary spike . I'm debating if that spike can happen at a specific place with in an enclosed space where there is less pressure in all other areas of that space .

Here's a pretty detailed discussion on these very same issues

http://thefiringline.com/forums/showthread.php?t=563332&highlight=secondary

[rsrocket1]

It certainly is an interesting thing to talk about, think about and contemplate. But just like many comedians used to say "I went to the doctor and said doc, it hurts when I do this. The doctor said 'then don't do it'".

Use the right speed powder for the gun you're shooting and you won't have to worry about "secondary explosions", "detonations", SEE's, etc.

[LynnJr]

I never heard of "secondary pressures."

There is a member of another forum that has not been civil to me since I suggested he should question some of the answers. He claimed he purchased a pressure tester that printed out graphs that read right then up as in time and pressure.

He used the same illustrations posted in the link. I suggested I could duplicate the reading; all I had to do was seat the bullet into the lands then sweat/worry if the bullet was going to get moving before the case head failed.

It was at about that time he got all snarky, he could not understand time and pressure. The graph looked normal and then one of those ‘and then moments’ occurred. Pressure had to build before the bullet setting against the lands could start moving. The part he could not understand was ‘the obstruction’, any delay in the bullet leaving causes the pressure to climb because we all know it is going to.

I am the fan of the ‘running start’ I want my bullet to have that ‘bullet jump’, I do not want my bullets to hesitate when it hits the rifling. I want my bullets to pass the rifling before it realizes it is there. Again, any delay in the exit of the bullet after leaving the case causes a spike.

F. Guffey

What a profound post.
Any delay in the bullet leaving causes a spike.
One must ask how much force does it take to push a bullet down a barrel? Can a Shooter use a cleaning rod to do this?
If there is a secondary spike it most likely comes from seating too deeply.
How much force is required to push a bullet out of a case? What happens when a bullet with excessive jump hits the lands?
If we look at the time where is the bullet when the secondary spike occurs near the chamber as Guffey suggests or nearer the muzzle?
120,000 psi show us the case.
And who is Snarky? My neighbors dog is called Sparky.

[fguffey]

The link is interesting for sure . I remember debating the secondary spike a couple years ago . Likely at the same forum and thread Guffey spoke of . At the time and what the link above seemed to suggest was that the secondary spike was localized to a specific spot in the bore . They also suggested a guy was able to predictably blow up a barrel at a specific spot using the secondary pressure spike .

I said they did not have a clue when it came to reading a graph, they had absolutely no ideal where the bullet was when the spike occurred and I said they had did not understand duration as it applies to time and distance.

F. Guffey

[fguffey]

Which again is interesting because you can almost see that happening . The thing is why wouldn't the pressure just back fill the bore behind the bullet rather then some how create a localized secondary spike ?

that is the reason I added Lynn Junior to my ignore list.

The localized spot informs anyone that can read a graph where the bullet was when the spike occurred. To reduce the spike give the bullet a running start. Again, I am the fan of 'the bullet jump', I want my bullets to hit the rifling with a running start. I am not entertained by the bullet sitting still against the rifling.

And if anyone is interested in seeing a spoke that is capable of producing enough pressure to crush a chase head seat an 8mm bullet into a 30/06 case. Then there is firing 8mm57 ammo in a 30/06 chamber. There will be a spike, the rifle could be rendered scrap but the graph will show a lull before the spike because the 8mm57 case will have to fill the larger 30/06 chamber before anything gets serious.

And then there was the shooter in North Texas that chambered a 308 W round in a 25/06 chamber and then pulled the trigger. Most were guessing how long the bullet was when it left the barrel; and I said had that happened they would still be looking for parts of the rifle.

F. Guffey

[LynnJr]

The above post is why the moderators of this forum should ban posters for life.
It is absolutely absurd to think the pressure will do anything but spike when using the wrong bullet for the bore as posted by Guffey.
If it wasn't we could simply seat it deeper as he already mistakenly suggested and it would hit the bore running.
What a bunch of dangerous jibberish to post on a reloading forum.

[Metal God]

The localized spot informs anyone that can read a graph where the bullet was when the spike occurred.

In this tread http://thefiringline.com/forums/show...ight=secondary

In post #13 Unclenick at "603Country" seems to show through the graphs and quick loads that the bullets of two different loads are at different distances down the bore even though the secondary spikes are in the same spot in mil-sec .

If that is true how can you tell where the bullet is in the bore based on the secondary spike ? I'll admit I don't know how to read the graph in that way .

In post #4 of the same thread Unclenick also says this

Some folks use "Secondary Spike" to refer to SEE (secondary explosion effect) or detonation, which is, indeed, rare. That's not what it refers to here, though. Here they are radial expansions that occur down barrel when the bullet has moved out faster than the powder can make gas to keep up with expansion. As it burns, though, it builds enough pressure behind its own mass to propel that mass to catch with and slam into the base of the bullet. The resulting radial expansion of the metal sets up a transverse wave that shows up in strain gauge readings at the chamber looking like a chamber pressure spike, even though there's actually no excess pressure at that location. What is seen is actually a reflection of the collision that momentarily stressed the bore radially further down. Instruments that read only chamber pressure, like copper crushers and piezo transducers, don't see the reflected wave, as their mounts tend to move with it. You have to use strain gauge instrumentation to pick it up.

Which seems to conclude that the pressure spike is not localized .

How ever later he says this

So, what are we seeing exactly? I don’t know. My current working theory is that when the powder mass catches up with the bullet, the bullet is upset outward during the attempt at sudden momentum transfer. If that stretches the bore radially even a little it will greatly increase the apparent friction. At that point pressure will abruptly straighten and push forward on the bullet base and bore. There are a couple of kinds of elastic event you can imagine occurring from that. One would be like Al’s Mode 6 on the harmonics page linked to above. Looking at how a long barrel deflects in his rifle animation, you can imagine that something pulling the barrel straight would crack a deflecting muzzle like a whip, which would be the other form. The problem is, I would expect an elastic event like that to ring, and not just stop and be damped out, the way the spike is. Though, of course, the stock bedding may do some of that damping, as might the receiver mass. Still, I’d expect some amount of reflection and ringing.

WOW mind blown ! So if I read that right . If the bore expands as the or when the secondary spike happens It can cause the bullet to yah in the bore slowing the bullet down do to the added friction resulting in a even more significant pressure spike .

Now couple that "theory" with a worn out bore that may even let the bullet be upset even more .

I'm not sure I'm buying it but I do love things that make you go hmmm ?

[LynnJr]

MetalGod
Thanks for those excerpts. I live 25 minutes from Varmint AL and I believe he was an engineer at Lawrence Livermore National Laboratory.
He modeled my 6 Dasher using FEA in one of his articles. A very bright guy.

[ruggyh]

PO Ackley ( Vol2) discussed this many years ago- secondary pressure has been documented.

Many ballistic labs have confirmed this condition.

attached is sample trace

Surprised, F. Guffey , how can you not of heard of this?

[LynnJr]

The reason he hasn't heard of it before is he is misquoting me in his post and most likely isn't able to correctly read the graph.
Once confusion set in he was toast.