Pressure Signs: What, Where and Why?

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Pressure Signs: What, Where and Why?

Post by Gwion » 20 Jan 2016, 8:42 am

As there are often questions about load data for various chamberings; often accompanied by warnings of 'approach MAX loads with caution'; i thought a thread on the all important signs that your action/chamber/brass are experiencing excessive stress through high pressure spikes was in order.

Now, i'm absolutely no expert on this subject, so i'll just start it off with my understanding of a few things that can cause changes in pressure and some pics of primers from gradually increasing loads. Please feel free to elaborate, correct or add to anything i write here. I'm wanting this to be a place for questions to be asked and experience to be shared.

Excessive pressures occur because there are either too many gasses produced by powder ignition and/or they can't escape the chamber in time to avoid placing cartridge, chamber or action (or all three) under undue stress. In extreme cases, this can cause something to go POP that isn't the bullet exiting the muzzle. This is never good for anyone but there are signs that all hand loaders should watch during load development to avoid excessive pressure spikes.

That pretty much exhausts my knowledge of the What and the Where. Now for the Why.

The most basic event causing excessive pressures is overloading the cartridge; that is, putting too much gunpowder in the case. This can be put down to exceeding MAX loads FOR YOUR FIREARM. The max load for your firearm may well be either above or BELOW the max loads listed in hand loading manuals.
However, sometimes using too little powder can cause spikes through what is termed FLASHOVER. This occurs when there is so little powder in the case that it lays flat in the case (for want of better terminology), the primer flash then ignites the top layer of powder momentarily, causing pressure in the chamber to rise. Then the rest of the powder ignites and pressures spike way beyond that which even a full (but evenly ignited) charge would create.

Excessive freebore, through seating bullets too deep, may also cause a similar effect but my understanding of this is limited so i wont go indepth. Basically i understand that pressures build, causing the bullet to leave the case but then as it engages the rifling, slowing down, pressure back up as the burn reaches it's peak.

What can be overlooked is changes in temperature. This can occur environmentally or circumstantially. One i have been guilty of is not accounting for a change in seasons. I developed a load during a crisp but clear Tassie winter week. I settled on a load about 0.4gn below the book max and everything was sweet at 1/2MOA accuracy and no real pressure signs. Come summer and some uncommonly hot days (for Tassie), rounds sitting out in the sun while i'm shooting and things behaved very differently, resulting in some very flat and cratered primers!
Similarly, if you have a hot chamber and insert your hottish but usually safe load, then take longer than usual to fire the round, the powder in the case may well have got pretty hot and changed the dynamics of the ensuing burn.

So, how do we avoid the pressure issue? Be careful, be cautious, be precise and be pedantic! Check, check, record and check.

Start by cross referencing load data from a number of manuals and study the spent brass while developing loads. Start your load development low in the numbers presented in your manuals and work up gradually. If you change a component or specification of your load; such as a new primer, new cases, new brand of same style bullet; decrease your charge and work it back up, just to be on the safe side! Also go gradually with changes in your seating depth, watching not just the groups for accuracy but also the cases for signs of a change in pressure.

So, what are pressure signs? Well, i'm pretty new to it all as well, I've only been reloading a couple of years. What i keep an eye on are the primers in my spent rounds. Look for gradual flattening of the primer and signs of "cratering" in your firing pin mark. If the round edges of the primer have been forced very flat to the edges of the primer pocket, or if the outer edge of your striker mark is raised and not smooth; your pressures are getting up pretty high and you should be VERY conscious that small changes in load specs or environmental/atmospheric/situational circumstances may well cause a pressure failure.

I will edit this post later with some pics of primers and maybe some more experienced reloading gurus can discuss what is and isn't a safe looking spent primer.

Can someone else discuss other signs of pressure?

Again, please feel free to correct any error or misunderstanding i have displayed above; and please; expand, elaborate or add to anything that needs mentioning.

If you're a newbie, ask questions so the experienced people can help you out.

Now i'm off to do some hand loading and testing.
Good luck, stay safe and have fun!
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Re: Pressure Signs: What, Where and Why?

Post by Chronos » 20 Jan 2016, 9:19 am

Great topic G, I have a little to share but I'll do it in seperate posts because I'm on my phone

In my experience primer cratering is a poor indicator of pressure. I've seen three rifles (2 Savages and 1 Remington) that showed major cratering on primers at low to mid range loads.

The reason may be 2 fold, soft primer cups and sloppy firing pin holes in the bolt face and or weak firing pin springs.

I do look for harder than usual bolt lift. Higher pressures generate more case expansion and can make a case harder to extract. lifting the bolt starts extraction and any case growth is noticeable here. Part of this sign is you start getting an imprint on the ejector on the case head. I'll post pics later of this

I also watch for primer flattening. If you do what's smart and work up from a mid range load (about 10% below max is a good guide) you'll be able to notice the difference in the corner radius of the primer. It starts going from a enter curve to a sharper corner or if yo like the flat face of the primer gets bigger. This can change if you change primers because some brands use thicker brass to make their primers, particularly "match" primers because match rifles tend to use heavier firing pin springs to reduce lock time.

Another thing you can measure is case head expansion. By measuring a fired and unfired case at the extractor groove with an accurate tool like a micrometer you'll see that a case expands when fired. They do expand to fill he chamber but they sping back to almost their original size. An acceptable case head growth is .001". More than that in one firing and you may be exerting excess img pressure on the case.

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Re: Pressure Signs: What, Where and Why?

Post by Gwion » 20 Jan 2016, 9:40 am

Good point about other causes of primer cratering, Chronos. It seems there are various views on the importance of cratering in checking for pressure signs. I have read articles that state it is a sure sign of increasing pressure but also other articles that support what you are saying re: varying causes of cratering.

The way i look at it, if my rifle usually has no cratering, even with moderate primer flattening, but then the primers are suddenly very flat and develop cratering, then i reckon i'm near max for my rifle and it's time to stop pushing the load and back off to find a node near by but a little lower in charge.
Would you say that is a fair way to look at it?
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Re: Pressure Signs: What, Where and Why?

Post by Wm.Traynor » 20 Jan 2016, 9:57 am

Ditto what Chronos says about bolt lift and ejector imprint.
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Re: Pressure Signs: What, Where and Why?

Post by Gwion » 21 Jan 2016, 10:35 am

Found a link to this video on another forum. Shows excessive pressure signs:

https://www.youtube.com/watch?v=orJdUR_X67M#t=302
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Re: Pressure Signs: What, Where and Why?

Post by ebr love » 10 Feb 2016, 11:12 am

Chronos wrote:In my experience primer cratering is a poor indicator of pressure. I've seen three rifles (2 Savages and 1 Remington) that showed major cratering on primers at low to mid range loads.


Ditto.

I've had two loads that I swapped between primers a few times on due to availability and with one brand they cratered on both and with CCI they were fine.
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Re: Pressure Signs: What, Where and Why?

Post by Wm.Traynor » 10 Feb 2016, 11:32 am

ebr love wrote:
Chronos wrote:In my experience primer cratering is a poor indicator of pressure. I've seen three rifles (2 Savages and 1 Remington) that showed major cratering on primers at low to mid range loads.


Ditto.

I've had two loads that I swapped between primers a few times on due to availability and with one brand they cratered on both and with CCI they were fine.


Ideally and IMO, you would reduce the powder charge when changing to a different primer.
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Re: Pressure Signs: What, Where and Why?

Post by Gwion » 10 Feb 2016, 12:34 pm

^^^ this is also what i have been lead to believe. Different primers have different flash signatures and will cause powders to ignite differently, creating the possibility of a different pressure signature; possibly higher pressure spike.
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Re: Pressure Signs: What, Where and Why?

Post by happyhunter » 10 Feb 2016, 3:04 pm

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Re: Pressure Signs: What, Where and Why?

Post by brett1868 » 10 Feb 2016, 6:16 pm

I'm guessing that having to use a mini sledge to open the action is a surefire sign of over pressure. I'll elaborate further once ADI respond to my email :)
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Re: Pressure Signs: What, Where and Why?

Post by Gwion » 10 Feb 2016, 7:09 pm

Always a worry if a mallet is part of your shooting kit!
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Re: Pressure Signs: What, Where and Why?

Post by deye243 » 10 Feb 2016, 9:16 pm

this is what you call excessive pressure

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Re: Pressure Signs: What, Where and Why?

Post by Redwood » 16 Feb 2016, 12:54 pm

Never seen anything like that last photo where the unfolded brass is still embedded :?
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Re: Pressure Signs: What, Where and Why?

Post by happyhunter » 17 Feb 2016, 10:45 am

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Re: Pressure Signs: What, Where and Why?

Post by Wm.Traynor » 17 Feb 2016, 1:40 pm

happyhunter wrote:Good article by Duane Siercks on what to look for with pics.
https://sierrabullets.wordpress.com/201 ... agnostics/


In both Cases #3 and #6, I would be suspicious of an Incipient Case Head Separation, even though the rings are higher up the walls of the cases, than I am personally used to. This applies if the cases were handed to me out of the blue and their rifles histories were unknown.
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Re: Pressure Signs: What, Where and Why?

Post by happyhunter » 17 Feb 2016, 3:37 pm

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Re: Pressure Signs: What, Where and Why?

Post by happyhunter » 17 Feb 2016, 4:18 pm

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Re: Pressure Signs: What, Where and Why?

Post by on_one_wheel » 17 Feb 2016, 9:01 pm

looking at the primer for signs of pressure is actually the least accurate way of determining excessive pressure.

Measuring for case head expansion using a micrometer is how big boys do it. ;)
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Re: Pressure Signs: What, Where and Why?

Post by COLLECTOR 1 » 17 Feb 2016, 11:20 pm

Im betting the pictures of the wrecked firearms and peeled back barrels are a result more of a "blocked barrel".....

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Re: Pressure Signs: What, Where and Why?

Post by Gwion » 18 Feb 2016, 6:27 am

on_one_wheel wrote:looking at the primer for signs of pressure is actually the least accurate way of determining excessive pressure.

Measuring for case head expansion using a micrometer is how big boys do it. ;)


So, what part of the case do you measure; just forward of the rim/head?
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Re: Pressure Signs: What, Where and Why?

Post by happyhunter » 18 Feb 2016, 7:52 am

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Re: Pressure Signs: What, Where and Why?

Post by on_one_wheel » 18 Feb 2016, 5:46 pm

Gwion wrote:
on_one_wheel wrote:looking at the primer for signs of pressure is actually the least accurate way of determining excessive pressure.

Measuring for case head expansion using a micrometer is how big boys do it. ;)


So, what part of the case do you measure; just forward of the rim/head?


The area immediately before the extractor grove.

Nick Harvey has a detailed writeup about this paticular method in his reloading manual.

I followed that method when I was in the quest for the hottest load my rifle could safely handle.

The end result was a load that goes so far beyond the typical reloading manuals max load that it's almost beyond belief.
Those super hot loads have improved accuracy because the increased velocity has reduced the affects of uncontrollable variables ... at the cost of barrel life of course.
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Re: Pressure Signs: What, Where and Why?

Post by Gwion » 18 Feb 2016, 5:59 pm

Cheers. I'll go back and re-read that section of th NH manual.
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Re: Pressure Signs: What, Where and Why?

Post by BBJ » 29 Feb 2016, 11:05 am

happyhunter wrote:The primers are cratered due to sloppy firing pin hole and on the pic below that you can clearly see a mark that looks like a bulge but after running through the FL die they are fine.


How does the firing pin hole translate back to cratered primers? Because the pin is too small and traps pressure?
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Re: Pressure Signs: What, Where and Why?

Post by happyhunter » 29 Feb 2016, 11:46 am

BBJ wrote:
happyhunter wrote:The primers are cratered due to sloppy firing pin hole and on the pic below that you can clearly see a mark that looks like a bulge but after running through the FL die they are fine.


How does the firing pin hole translate back to cratered primers? Because the pin is too small and traps pressure?


The clearence between the firing pin hole and firing pin is excessive and allows primer cup metal to flow into the space.
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Re: Pressure Signs: What, Where and Why?

Post by Gregg » 16 Mar 2016, 2:28 pm

on_one_wheel wrote:Measuring for case head expansion using a micrometer is how big boys do it. ;)


Likely not many people with one lying around however.
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Re: Pressure Signs: What, Where and Why?

Post by happyhunter » 16 Mar 2016, 9:35 pm

on_one_wheel wrote:looking at the primer for signs of pressure is actually the least accurate way of determining excessive pressure.

Measuring for case head expansion using a micrometer is how big boys do it. ;)


Or anybody with the Hornady manual. You need one that does .0001". Big boys don't handload.
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Re: Pressure Signs: What, Where and Why?

Post by Norton » 10 Jun 2016, 11:09 am

happyhunter wrote:Big boys don't handload.


Huh?
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Re: Pressure Signs: What, Where and Why?

Post by Gwion » 10 Jun 2016, 3:32 pm

Norton wrote:
happyhunter wrote:Big boys don't handload.


Huh?


Apparently big boys guns shoot best with factory ammo??? :unknown: :unknown: :unknown:
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Re: Pressure Signs: What, Where and Why?

Post by Chronos » 10 Jun 2016, 5:11 pm

Wm.Traynor wrote:
happyhunter wrote:Good article by Duane Siercks on what to look for with pics.
https://sierrabullets.wordpress.com/201 ... agnostics/


In both Cases #3 and #6, I would be suspicious of an Incipient Case Head Separation, even though the rings are higher up the walls of the cases, than I am personally used to. This applies if the cases were handed to me out of the blue and their rifles histories were unknown.
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I reckon the "smith" who chambered those rifles needs to go back to the loading dock where his skill set is probably more suited.

They look clearly like pick up on the reamer has marked the chamber and on firing the brass simply flows into the groove. I guess repeated f/l sizing and firing could work harden that area and eventually cause a failure but I think it might take dozens of firings at least

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