Understanding Pressure

Posted by: orkan

Understanding Pressure - 07/19/16 07:13 PM



Primal Rights, Inc - Understanding Pressure

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!
Posted by: SlickerThanSnot

Re: Understanding Pressure - 07/19/16 11:23 PM

thank you for doing that article. has great pictures and explanations. will come in handy for future reference.
Posted by: Plant.One

Re: Understanding Pressure - 07/20/16 12:35 AM

thats a wondeful writeup - hopefully you get lots of google hits with that.


i remember reading on another forum where a fella said that blown primers were just fine, as long as there were only "a few here and there". IIRC he got offended when i asked him where he shot so that i'd avoid that range so i could never end up on the bench next to where he was shooting.
Posted by: reaper4

Re: Understanding Pressure - 07/20/16 02:38 AM

Great write up and it really illustrated clearly signs I should be looking for as a relatively new reloader. I hear things and have a hard time determining exactly what that looks like on a case and now I know. I have stayed in the lower charge weights and still seen ejector print on cases and had to ask an older more experienced reloader what that meant. Thanks for the great info
Posted by: 7887mm08

Re: Understanding Pressure - 07/20/16 06:43 AM

Seeing is believing! Thank you
Posted by: Lefty SRH

Re: Understanding Pressure - 07/20/16 07:06 AM

Good article, thank you.
Posted by: mbaysinger89

Re: Understanding Pressure - 07/20/16 08:38 AM

Great article.
Posted by: vanhornet

Re: Understanding Pressure - 07/20/16 10:23 AM

Excellent write-up. Thanks for posting.
Posted by: dan158

Re: Understanding Pressure - 07/20/16 04:25 PM

Lets say one shut off the gas system so bolt wont move for an AR.
One ejects cases manually by pulling the BCG back.. One has no swipe marks and brass looks fine.. But when gas system is running it leaves bad markings. Is it considered over pressure or just the timing at which can not be slowed down enough to function as a semi auto? There would be more time for the brass to cool down by manually eject with gas off than if being ripped out so fast before has a chance to cool and shrink.. Over pressure or not?? Then what would the signs be? The same as a bolt rifle then?? Thanks. Dan
Posted by: orkan

Re: Understanding Pressure - 07/20/16 04:53 PM

Originally Posted By: dan158
Lets say one shut off the gas system so bolt wont move for an AR.
One ejects cases manually by pulling the BCG back.. One has no swipe marks and brass looks fine.. But when gas system is running it leaves bad markings. Is it considered over pressure or just the timing at which can not be slowed down enough to function as a semi auto? There would be more time for the brass to cool down by manually eject with gas off than if being ripped out so fast before has a chance to cool and shrink.. Over pressure or not?? Then what would the signs be? The same as a bolt rifle then?? Thanks. Dan
I'm sorry but I have not done adequate testing to confidently answer your question, and I specialize in work with bolt action precision rifles.

My speculation would be that the gas system is part of the "expansion chamber" in a semi-auto, as is necessary for proper function in a gas-driven system. In order to read pressure signs effectively, the entire system must be functioning as designed. It is no surprise that bolt action rifles are considered to be much easier to work with, as this variable does not exist. There is no cyclic "timing" to worry about with bolt rifles. While I'm certain that the pressure shown in your hypothetical should be viewed as "real," I have not done adequate testing to tell you why it appears in semi-auto mode and not when the bolt is remaining locked. Perhaps someone with more experience with semi-auto's can comment.
Posted by: GLShooter

Re: Understanding Pressure - 07/20/16 05:30 PM

You would be looking at case head expansion and primers on the home front along with chronograph numbers on the manual; operated rifle just exactly like a bolt action rifle. The gas system is opening up when there is still pressure on the cartridge effectively turning it into a rocket hitting the bolt face ripping on rims and pounding the brass down into the ejector hole. The brass is hot and very elastic at that point. Hence the fact that you can fire ten rounds in the same AR chamber and get three or four different shoulder height readings. The earlier it opens, as in over gassed, the more violence to your case heads will be observed. The adjustable block or a heavier spring/bolt combination will mitigate some of these issues of observed over pressure indicators. Also be aware that a large firing pin tunnel in concert with a small firing pin will give false signs of pressure as this will induce cratering. Modern Remington BA rifles show this all the time with KNOWN lower pressure loadings.

Greg
Posted by: cherokeetracker

Re: Understanding Pressure - 07/20/16 06:30 PM

Orkan Good Article, WTG.
Posted by: Rusty Black

Re: Understanding Pressure - 07/20/16 08:53 PM

Great article....thankyou
Posted by: Kermit in Va.

Re: Understanding Pressure - 07/23/16 09:11 AM

Thanks...
Every reloader should see this.


Kermit
Posted by: ackleyman

Re: Understanding Pressure - 07/23/16 10:20 AM

Best article that I have ever read on pressure! Thanks for putting so much time and effort into this article!
Posted by: 204 AR

Re: Understanding Pressure - 07/23/16 12:03 PM

Ditto what ackleyman said!
Posted by: Bowhunt

Re: Understanding Pressure - 07/24/16 09:29 PM

Fantastic article! Great job and thanks so much for posting.
Posted by: jpx2rk

Re: Understanding Pressure - 08/10/16 03:44 PM

Good read. At what level of magnification were the pictures taken? Can you see most or all of the danger signs with a 4x magnifying glass, or is stronger magnification needed??
Posted by: orkan

Re: Understanding Pressure - 08/12/16 08:25 PM

The pics were taken with a special "macro" lens on my DSLR. Here is the lens: http://amzn.to/29KFZH8

You can usually see pressure signs with the naked eye. The images were high resolution so that the effects could be easily seen by anyone reading. Any magnification will help however. The illuminated loupe I mention in the article will work quite well. Here it is: http://amzn.to/29KGAs4
Posted by: Mark204

Re: Understanding Pressure - 08/29/16 05:16 PM

Wow...written in a way that "average folks" can understand. The pics were essential in getting a grasp on what you were trying to convey. Thank you!!
Posted by: Jason B

Re: Understanding Pressure - 10/07/16 09:48 AM

Great Article.

I will echo what Mark204 said in that it was written for the "average folk". The pictures made it very easy to follow and understand.

Thanks orkan!!
Posted by: why

Re: Understanding Pressure - 10/08/16 09:24 PM

I didn't read this untill today. Very informative, I learned a few things. Thank you.
Posted by: SteWil234

Re: Understanding Pressure - 10/12/16 01:26 PM

Thank You, some very helpful information.
Posted by: whispig

Re: Understanding Pressure - 10/21/16 08:57 PM

Great read. Just goes to show how much I don't know... VERY much appreciated.
Posted by: Nitroman

Re: Understanding Pressure - 11/08/16 09:59 PM

Temperature increases the RATE of a chemical reaction, which is why loads that work on a 40* day will extrude brass on a 110* day in the sun, not the AMOUNT of energy.

Looking at that Mauser bolt that had the lugs sheared off; unfortunately people aren't very smart these days, lots of stupid going around, and they do not read the loading manuals and load to velocity, they load to what is in the book and wonder why things go wrong.

The only way to truly know what is going on inside your rifle is to use pressure testing equipment such as Pressure Trace and a good chronograph like the Oehler 35p.

Good article.
Posted by: NV Bill

Re: Understanding Pressure - 11/09/16 09:01 AM

Well done article, thank you for taking the time to put all that info together with photos.
Posted by: Wyodogger

Re: Understanding Pressure - 11/14/16 10:14 PM

Very helpful article!

BTW the blown-up rifle belonged to Chris Comer (RIP), a shooting buddy, a vet whose father died in WWII on Peleliu, and one of the nicest men you'd ever want to know.

Fulton armory said the barrel was made of a bad batch of steel that was held at too high a temp too long. Consequently, the steel molecules were too big, which eventually caused the blow-up. It was not a high pressure load. That was also shown by looking at the primer of the round in the chamber when it blew up; no pressure signs.

To add insult to Chris's injury, he couldn't replace the rifle as it was grandfathered under one of Kalifornia's goofy AW laws. I'm so glad I left that socialist, leftist hellhole.
Posted by: sakalmon

Re: Understanding Pressure - 11/21/16 08:02 PM

Awesome! I will reread several times until it all sinks in.
Posted by: prairiefire

Re: Understanding Pressure - 01/01/17 02:11 PM

For a relative reloading newly (only been at it for three years) this was an eye opening article. Made me take a step back and realize how complex accurate reloading can be and how one must never become complacent. Thanks!!
Posted by: Strudy68

Re: Understanding Pressure - 01/17/17 05:12 AM

Very good read!!! nice to see to step by step surprised that more pressure didn't have the effect that one would think....
Posted by: GhostsoftheGrass

Re: Understanding Pressure - 01/19/17 04:33 PM

Very good! Thank you, sir!
Posted by: Bauxite

Re: Understanding Pressure - 03/20/18 06:07 AM

Let me say thank you, too. Excellent.
Posted by: Wa_Coyote_Hunter

Re: Understanding Pressure - 08/02/18 12:03 AM

nice read saved on my computer for future reading. got a 357 mag from my uncle 30 years ago. with some reloads. fired a few rounds then bang pierced a primer. the sound of the shot sounded like it doubled. disposed of the rest of those rounds in a hurry.
Posted by: Predzys

Re: Understanding Pressure - 11/27/18 11:08 PM

Nice write up
Posted by: Flesh Eater

Re: Understanding Pressure - 01/23/19 09:14 PM

Very good article!

Iím thinking about getting into reloading and this was an eye opener even about factory ammo! Which after reading it I checked some casings on my once fired .17 Hornet brass. Sure enough, the Hornady 25 grain JHP was hot enough to make the primer look like your early pictures of cratering.

My question about the oil is how do you deal with rifles that get ice all over them, then sweat when brought in? Oil seems unavoidable.
Posted by: Rock Knocker

Re: Understanding Pressure - 01/23/19 10:25 PM

Originally Posted By: Flesh Eater

My question about the oil is how do you deal with rifles that get ice all over them, then sweat when brought in? Oil seems unavoidable.


When I get home from hunting in the cold, -15 to -20 at times. I ether leave the gun in the case until it's all at room temp or I take the gun out, dang thing freezes solid, literally, but field strip the gun as much as I can and place the parts over some of my furnace vents and that warms it up and drys it off pretty quick. Plus not having enough lube for it all to be flowing around helps also.
Posted by: Flesh Eater

Re: Understanding Pressure - 01/24/19 09:23 AM

Originally Posted By: Rock Knocker
Originally Posted By: Flesh Eater

My question about the oil is how do you deal with rifles that get ice all over them, then sweat when brought in? Oil seems unavoidable.


When I get home from hunting in the cold, -15 to -20 at times. I ether leave the gun in the case until it's all at room temp or I take the gun out, dang thing freezes solid, literally, but field strip the gun as much as I can and place the parts over some of my furnace vents and that warms it up and drys it off pretty quick. Plus not having enough lube for it all to be flowing around helps also.


I've noticed my rifles seemingly dry up within an hour, but they sweat like crazy during the process. I assume if it's sweating on the outside of the barrel, it has to be sweating inside, right?

Do you oil your chamber at all? Would oiling it, then wiping it with a dry rag, or dry swab create the same pressure issues as discussed, or is that usually caused by an excess of oil?
Posted by: JMette

Re: Understanding Pressure - 08/29/19 11:57 PM

Velocity =Ďs pressure. That helps a lot if you know roughly where your rifle should be shooting.
Posted by: orkan

Re: Understanding Pressure - 08/30/19 12:00 AM

Originally Posted By: JMette
Velocity =Ďs pressure. That helps a lot if you know roughly where your rifle should be shooting.


To clarify, are you saying velocity is an indicator of pressure?
Posted by: JMette

Re: Understanding Pressure - 08/31/19 12:56 AM

It could be. Itís the closest thing we have to measuring pressure. Nothing is the same in reloading. Everything is different and has variables. But if a safe pressure in a manual states a given FPS with a certain grain. It may take your rifle 1 grain more powder to achieve them same results or it may take your rifle 1/2 grain less. If youíve got a loose barrel you wonít have pressures therefore you wonít have the velocities. If youíve got a tight barrel youíll have increased pressure and therefore youíll get an increase in velocity with a lower powder charge

Hope makes sense.
Posted by: JMette

Re: Understanding Pressure - 08/31/19 01:00 AM

So essentially, yes velocity = pressure. Higher the pressure the more the velocity. Lower the pressure the lower the velocity. Only thing weíve got to measure pressure is to judge by book velocity. Primer crater, head swipes, flatten primmer, can all show false pressures. And no signs doesnít necessarily mean no high pressure either.
Posted by: Bob_Atl

Re: Understanding Pressure - 08/31/19 10:48 AM

An old thread !

Need to be careful on the oversimplification of MUZ-V == PSI.

To illustrate, some powders will give a spike of pressure, above max PSI,
while other powders will give a safe smoothed mound of pressure for same MUZ-V.

A handy tool to see examples of estimated pressure curve shape is Quickload.
Posted by: JMette

Re: Understanding Pressure - 09/01/19 09:49 PM

I use quickload frequently. Itís a broad analysis about velocity but itís true mostly to an extent.
Posted by: orkan

Re: Understanding Pressure - 09/03/19 01:13 PM

JMette, I can not agree with your assessments here. Velocity is a result, and while some correlation exists, correlation does not imply or equal causation.

Especially in this case, because as Bob Atl said, pressure is pressure, and is not velocity specific in any instance. This is easily evidenced by the same loads being able to produce 100fps more or 100fps less in different barrels. So while velocity will be a metric for the median, to either side of the median is danger.

Posted by: Don Fischer

Re: Understanding Pressure - 09/29/19 12:59 PM

Every thing you need to now about pressure. To much can blow up any rifle and do a lot of damage to you. Doesn't matter what the pressure is as the overwhelming majority of us have no means to measure it. That mean's pressure is just a number! What is important is to learn to recognize pressure sign's and how to avoid them. Then suddenly the number means nothing anymore because you'll never know what your getting in the first place. What you need to know is how to recognize dangerous pressure when you see it!
Posted by: Whisky Tahoe

Re: Understanding Pressure - 05/25/20 05:06 PM

Here are a couple of things to consider regarding pressure in addition to the excellent article from Primal Rights.

1) Case expansion. By measuring your cases above the web, you can track expansion as a result of pressure. Case expansion can be normal especially in short cases like the 6BR where the die isn't sizing the entire length of a case. Over time this leads to "clicky" bolt. A sign that the case is hard to pull out by the extractor. Rifles are all different so signs like ejector wipe may not appear as serious from rifle to rifle. Measuring case expansion can give you an objective way to detect pressure. Small body dies can be used to size the brass prior to full length resizing if you have "expanded" a few cases with a high pressure load. Case heads with wipe signs don't clean up as easy and hang around potentially leading people to believe they still have high pressure.

2) Pierced primers. Primers can be pierced by over pressure loads certainly but can also be pierced by bolts with large firing pin holes and small firing pins. Savage bolts are often built with small diameter pins and large holes. If your cartridge has small primers this can easily occur. Best remedy is to have the bolt "bushed" where the firing pin hole is reduced. I had this problem with a new to me used rifle that was piercing low pressure loads. I even tested some factory ammo to be add another data point. Sent the bolt to a pro to get bushed, problem solved.

"The more you know"...
Posted by: Jason50cal

Re: Understanding Pressure - 05/26/20 09:41 PM

Great article. Thank you
Posted by: crschum

Re: Understanding Pressure - 01/12/21 03:12 PM

Wish I would have had this around when I started reloading when I was in college. Would have made me a little less skiddish with loading to the point I could really utilize the brass through its entire life. Great article!
Posted by: ILPredHunter12

Re: Understanding Pressure - 07/31/21 05:22 PM

late to the party here, but great read! thank you!