Understanding Pressure

[Orkan]

Just finished up an article that should help people gain a better understanding of the pressure at play in their firearms.

Let me know if you have any questions and I look forward to your comments!

 

[Reloadron]

I fond it to be a good, well written article. The article was well supported with images and I like that. One picture really is worth a thousand words.

Ron

 

[Grumulkin]

It is a good article. Consider this quote from the article:

"Excess pressure can be detected fairly reliably by use of a chronograph as well. If you are adding powder incrementally such as when doing an OCW load development process, yet rounds with more powder yield less velocity, this can be an indication that the brass has been pushed beyond it's limit. If it can not hold the pressure, then it will not constrict the volume adequately and will result in less pressure being exerted on the bullet. The pressure will be lost to component expansion internally. A fairly precise chronograph is required to be able to track this event, but it is noteworthy. Usually other pressure signs will be evident as well if this velocity loss is taking place, thus it is not a requirement to use a chronograph. Though it can be helpful as an indicator."

I disagree with part of this statement but it's true that more powder can result in decreased velocity. What I disagree with is that the velocity loss is from less pressure or to component expansion internally.

What would more powder give less pressure? It doesn't make sense at all. What I think happens is that there is a very high pressure spike that is there and gone before the bullet has time to get optimal acceleration. That's something like the mechanism of the bent op rods on Garands when using too slow a powder.

As for the statement about component expansion internally (which I presume means inside the cartridge); that's a bunch of nonsense. At acceptable pressures, a case expands maximally. At excess pressure, something else has to give and that would be the bolt and/or chamber.

In my perusal of the article (forgive me if I missed it), I didn't see any mention of group size or location. As a load get above the optimal pressure group size will many times expand markedly. Another sign is a sudden shift in group location.

 

[Orkan]

What would more powder give less pressure?

I think you may have misunderstood what I wrote there. I do not say that more powder gives less pressure. What I said was that there was less pressure available to act on the bullet, due to component expansion. The pressure is not lower, but rather is lost to case expansion. This very same behavior can be exhibited when forming AI brass. Moving the brass in such a way tends to absorb pressure.

My findings are easily reproduced and evidenced by the fact that over-pressure rounds tend to exhibit more case web expansion. If you perform similar testing, you will find that the cases measured at the point of velocity loss will be larger and exhibit less spring-back than cases fired before velocity loss.

 

[LiveLife]

Thanks for the article.

 

[Grumulkin]

I think you may have misunderstood what I wrote there. I do not say that more powder gives less pressure. What I said was that there was less pressure available to act on the bullet, due to component expansion. The pressure is not lower, but rather is lost to case expansion. This very same behavior can be exhibited when forming AI brass. Moving the brass in such a way tends to absorb pressure.

My findings are easily reproduced and evidenced by the fact that over-pressure rounds tend to exhibit more case web expansion. If you perform similar testing, you will find that the cases measured at the point of velocity loss will be larger and exhibit less spring-back than cases fired before velocity loss.

The pressure inside the case is going to exert exactly the same pressure on the bullet as it does on the case. Said pressure will expand the case, start the bullet moving down the bore, exert pressure on the bolt face, etc.

Think about it, if pressure is increased too much, something has to give, or if you prefer, absorb the pressure. Perhaps the chamber fails sending metal shards around, perhaps the bolt fails. Of course, the case most likely will fail before a chamber will. At this extreme, the bullet moves hardly at all and yet for an instant the pressure on the bullet was exactly the same as the pressure exerted on the inside walls of the case.

And yes, it makes sense that cases measured at the point of velocity loss will be larger but it still holds that you can't have one pressure exerted on the inside walls of the case and a different one on the bullet.

 

[spitballer]

Thanks Orkan for excellent article: the image of the sheared bolt lugs was especially startling ("waiting right there is your face..."). This piece is written in a no-nonsense style and conveys information graphically and authoritatively, which is appropriate for this subject IMO.

 

[Orkan]

And yes, it makes sense that cases measured at the point of velocity loss will be larger but it still holds that you can't have one pressure exerted on the inside walls of the case and a different one on the bullet.

I'm sorry, but the science behind this is quite settled.

If the space containing the pressure becomes larger, then the amount of pressure will be less, all other things being equal. Thus while pressure starts out higher than the previous charge weight, the bullet is not being acted on by more pressure due to the fact the space behind the gas seal of the bullet is greater, and also being absorbed by the work of expanding the brass. So it is not possible for you to concede that the brass grows, yet deny that less pressure is acting on the bullet.

Fully explaining this concept in detail was outside the scope of this article. There are many books on the subject however.

 

[Walkalong]

The pressure inside the case is going to exert exactly the same pressure on the bullet as it does on the case.

It makes sense that the pressure would be the same on all surfaces of the "container, except for the fact the bullet starts moving. I don't know how that affects things. It must start at the same pressure, and as pressure subsides, why would it not over all the surfaces, chamber, barrel, bullet.

I would be interested in the explanation. Not that it's really important. All we need to know is our pressure is at a safe level.

 

[Orkan]

It makes sense that the pressure would be the same on all surfaces of the "container

Indeed you are correct. However, if the "container" expands, the pressure is then reduced, which results in lower velocity.

 

[Grumulkin]

In a bomb, pressure increases and the container expands until a part of it fails and releases the pressure.

In a gun, pressure increases and is usually relieved by the bullet moving out of the case and down the bore. In an overload, pressure increases too fast for the bullet to compensate by moving down the bore. The pressure is relieved by case expansion, pressure against the bolt face which will give a little, may be a pierced primer that releases some gas, etc. or maybe the chamber itself fails. Rather than saying velocity falls off because pressure decreases (actually pressure is very high), it would be more accurate to say velocity decreases because pressure is relieved in ways other than the bullet moving down the bore.

The case always expands when a cartridge is fired. With excessive pressure it expands a little more but not enough to explain the velocity decrease all by itself.

 

[Orkan]

Rather than saying velocity falls off because pressure decreases (actually pressure is very high), it would be more accurate to say velocity decreases because pressure is relieved in ways other than the bullet moving down the bore.

A direct quote from the article:

Velocity Loss
Excess pressure can be detected fairly reliably by use of a chronograph as well. If you are adding powder incrementally such as when doing an OCW load development process, yet rounds with more powder yield less velocity, this can be an indication that the brass has been pushed beyond it's limit. If it can not hold the pressure, then it will not constrict the volume adequately and will result in less pressure being exerted on the bullet. The pressure will be lost to component expansion internally. A fairly precise chronograph is required to be able to track this event, but it is noteworthy.

It says the exact same thing that you are now saying. I'm sorry but I know of no other way to explain this to you, nor do I think any explanation I could provide would be adequate for you. I'll say no more on it.

 

[hdwhit]

I really appreciate you including images for indicators that appear to suggest high pressure but which are not necessarily abnormal. It is very important that people be able to distinguish and ignore false positives.

 

[Walkalong]

Indeed you are correct. However, if the "container" expands, the pressure is then reduced, which results in lower velocity.

But isn't it reduced on all surfaces?

 

[243winxb]

Volume vs Pressure

5. Live Gun Firings, starting page 26. http://www.dtic.mil/dtic/tr/fulltext/u2/a456635.pdf I find the 4% difference in case volume interesting, when comparing new to fully expanded brass in the tiny 5.56 cartridge. I won't pretend that i understand the rest of it. The charts do show when the bullet moves.

 

[R.W.Dale]

Here's the problem I have with the articles presentation

New loaders especially like to take the whole notion of "pressure" signs as somehow correlating to SAAMI limits. That somehow a 308 case head at just over 60k will magically start showing these indicators ignoring the fact that the same basic piece of brass in the form of 7x57 is supposed to do the same at a mere 51k

These pressure signs (guesses as I call them) aren't a case of "oops! That's a tad hot" but rather STOP reevaluate everything. And then that's assuming you see any of these at all which you may not as the individual chamber has as much to do with any of these as the actual pressure.

 

[Hangingrock]

If you do not have the proper instrumentation and procedures to measure pressure you are making assumptions which may not be valid. More than a chronograph is required to ascertain correctly the forces applied.

 

[JimKirk]

But isn't it reduced on all surfaces?

In my opinion it does as the pressure it equal in all directions ... if reduced then it would be reduced in all directions ...

the Scarey part of pressure is that it finds the weakest path to escape or try to excape.... which we all hope is down the barrel ....not out through the brass or primer/ web area .... or out through the metal of the gun....

I noticed the term "small explosion" occurs .... I believe that the term "controlled burn" would be a more exact description .... although a short lived burn....

Good photos ...

 

[Slamfire]

If the brass is not allowed to grab hold of the chamber walls, then all of that pressure is transferred rearward onto the base of the case, the bolt face, and subsequently the bolt lugs and action. If water is in between the brass and the chamber walls, even very light and mildly loaded ammunition will cause severe pressure signs. The ability of the brass to grab onto the chamber walls is paramount to the proper functionality of the firearm.

I disagree with this. You are citing authority and authority is wrong. This in fact, comes from a Army cover up of the failures of the 1921 tin can ammunition. The Army created a bore obstruction with the tin coating on the bullet, that raised pressures and blew up rifles. Instead of acknowledging the problem they created by coating the bullets in tin, the Army blamed a shooter practice: shooters were greasing their bullets. Everything you have stated came from this cover up and decades of amplifications of this cover up, and it is all untrue. The locking mechanism of any firearm is designed to contain the full thrust of the cartridge ignoring any case to chamber friction. Water, grease and oil do break the friction between case and chamber but the action is designed to hold the thrust as long as the ammunition meets pressure requirements. Exceeding pressure limits on a cartridge over stresses the action and the barrel, and this is true whether the cartridge is lubricated or not.

In fact, breaking the friction between case and chamber increases function reliability in all automatic or semi automatic mechanisms, such as the 276 Pedersen rifle:





The Oerlikon action used greased ammunition:




Ordnance Pamphlet 911, page 105

http://archive.hnsa.org/doc/gun20mm/part4.htm#pg105



GREASING AMMUNITION

All 20 mm. A.A. Mark 2 and Mark 4 ammunition MUST BE COMPLETELY COVERED WITH A LIGHT COAT OF MINERAL GREASE BEFORE BEING LOADED INTO THE MAGAZINE.

The ammunition is usually packed greased. However, this grease tends to dry off. Whether cartridges are packed greased or not, they should be regreased before loading the magazine.


NOTE-A small amount of mineral grease, applied shortly before firing, to the cartridge case that is visible in the magazine mouthpiece, will assist in preventing a jam in the gun barrel.

Dry ammunition or ammunition with insufficient grease will jam in the gun chamber when fired and extraction will be very difficult, if not impossible. See Page 110 for use of torn cartridge extractor.

The chamber flutes in roller bolt actions are there to specifically break the friction between case and chamber. The action will not function with a high level of friction between case and chamber. These chamber flutes are the reason you don't see oilers on gun mechanisms anymore. Oilers were very common prior to WW2 but that was almost 80 years ago and shooters don't know their firearm history. Functionally, chamber flutes do the same thing as oilers or manually greased cases, that is break the friction between case and chamber, and do so without requiring the shooter to lug around oil bottles to fill reservoirs. Hence, oilers went away.





I have shot tens of thousands of 308 rounds coated with paste wax, which acts similar to the 276 Pedersen lubricant, and I have shot thousands of cases which were greased, oiled, or coated with some lubricant. And I have shot tens of thousands of greasy 22 LR ammunition in competition. To date no pressure problems or indications of pressure.

I ran a greased bullet test. The huge grease blobs around the bullet and shoulder did not cause pressure problems and it can be clearly seen, the grease moved down the shoulder, down the sidewalls, filled the extractor groove, and was squeezed into the action. I had zero pressure problems. The load was 168 gr Nosler Match 47.0 IMR 4895 thrown lot L7926 LC53 WLR (brass) OAL 3.30" fired in a 30-06 rifle. This is a full power match load and yet I experienced no pressure issues, indications, nor was velocity appreciably different from the same load fired dry.



Grease was squeezed between the case and chamber out the back into the action area. This is a positive indication that the entire case was floating in grease during combustion.

 

[Grumulkin]

That's quite some grease test. Not one I'd have done but I'm glad someone other than I boldly went where not many, if any, have gone before.

 

[Walkalong]

I disagree with this.

One part of it is definitely right, the brass must expand and seal the chamber from hot high pressure gases so they only escape down the barrel, and not through the action into our face.

Dry it will grab early and stretch in the rear, lubed it will slide back first and eliminate or reduce that stretch, but in both cases the pressure blows the forward part of the case walls out and they seal the chamber. It doesn't have to be 110%, but it has to be enough for long enough.

 

[fouled bore]

I am glad to see I am not the only one to have an issue with this part. The thin brass that a cartridge is made from has no ability to contain the pressure from a 50k psi cartridge. No matter if the cartidge or chamber is lubed or not the breach face must be able to contain the pressure from the cartridge.

 

[macgrumpy]

The pressure inside a chamber is produced by the energy released by the burning powder. The energy released by the powder is simply heated molecules.

If enough energy is produced to exceed the tensile strength of the component parts of the system (cartridge case, primer, bolt lugs, receiver lugs, etc.) then things will start to stretch.

As they stretch they use up more energy than is normal for the system (the heat energy is used up in expanding the parts).

Using more energy to stretch metal leaves less energy to produce pressure.

So not only do you have an expanded pressure vessel but you are using up the limited amount of energy (produced by the measured amount of powder) to expand the pressure vessel, both of which will result in a reduced amount of energy left to produce pressure.

 

[243winxb]

Why velocity drops when pressure has become to high.

Brass has to spring back while bullet is still in the barrel. If no spring back, larger volume, less pressure, velocity drops off. The 5.56 has a 4% volume increase. The 308 win has 8% ( so i am told). What does Quickload say when the volume is increased by 4 or 8% ?

 

[Reloadron]

Years ago I had a 7.72 x 51 NATO cartridge which cooked off in a fire. Came off the Edson Range at Camp Pendleton. It was a nice desk ornament. The live round found its way into a burn barrel on the range. Now we all know what happens when a round cooks off in a fire. The brass never split and the bullet or projectile if you prefer exited the case with a very low velocity. The cool part was the case bulged to where it looked like a large cat's eye marble midpoint.

What happened was pretty obvious, the powder ignited or the primer cooked off igniting the powder. Pressure began to build and when something had to give the bullet exited the case. The case was not confined to a chamber so it had room to expand due to the building pressure. The weakest point was the bullet to case and pop goes the bullet. Now I am not suggesting that anyone toss a handful of .308 loaded ammunition in a fire but I am sure we get the drift. I am also sure the pressures reached were nowhere near the maximum SAAMI 308 Winchester pressure of 62,000 PSIG. The case has a volume and as the pressure increased the case volume increased right till the bullet exited the case at which point the pressure dropped to zero. So the only reason that a cartridge like the 308 Winchester can acheive a chamber pressure of 62,000 PSIG is because the cartridge is contained in a chamber.

When looking at chamber pressures I prefer to instrument the rifle and actually look at the pressure or more important the entire pressure curve which when measured and recorded they resemble this:

While the above image is software generated an actual gauge measured curve would likely look about the same. When the primer ignites the powder charge things start happening real fast. Keep in mind this is a .223 Remington example and the data is on the chart. This image actually came from here on the web. That said look at the curve. The pressure builds rapidly traveling upward till it peaks. That peak is about 0.3 milli-Second or 300 micro-seconds into the time line. The bullet exits the case. As the bullet travels down the barrel the volume is increasing for the gas we have so with more volume the pressure begins to drop. I guess if you are into it figure the barrel volume with the lands and grooves and you will know how much per inch the volume increases. Once the bullet leaves the barrel the pressure drops to zero PSIG.

When things go correctly the case expands and forms a seal, all of the pressure should be pushing our bullet down the barrel. Again, less a chamber the cartridge would simply pop the bullet out and likely expand some before the bullet exited the case. The one I had also blew out the primer. Someone swiped that cool case I had when I was a Marine Corps Recruiter using it as a desk ornament.

<EDIT> In the forever it took me to type and post this I see 243 winxb beat me to it with a pressure curve chart. </EDIT>

Ron