Pressure Signs

[BrentM]

I came across this issue last weekend while shooting a 7 - 08. Things I look for in load development are as follows:

Primer flattening and cratering
Sticky bolt lift on up cycle and recycle
Extractor hole mark/ring

Some times I have seen primer issues without the rest of the signs and don't get all worked up over it. Well, unless the primer is just hammered by pressure. I tend to get much more concerned when I see extractor marks and bolt lift issues.

So, the issue came when I shot some PPU ammo. A little cratering, no bolt lift issue, but I did see and extractor ring mark. At max grains of 4064 I did not see signs on this PPU brass. I think I can go up 1 grain more but have not tested it yet. I don't recall ever seeing extractor marks on factory ammo. Is this common? Am I reading too much into it?

 

[varmintH8R]

I tend to evaluate similar to you - primer watching can be fickle at best. Marks on the brass and sticky bolts are a no-go area for me.

I think it is weird to see extractor marks on factory ammo, but I have shot some factory fodder that I know is hot. They are more interested in velocity claims than brass life, that is for sure. Bottom line, I wouldn't use your experience with that factory ammo as any type of benchmark for your own loading.

I am a "never exceed book max" kind of guy. If you are at book max with your current load MHO is why push it? The extra velocity is probably worthless by any practical measure.... I know other guys view it differently but to me it is pretty simple risk vs benefit.

I would be interested in what the manufacturer would say if you emailed a pic and asked their opinion? Just curiosity on my part.

 

[BrentM]

I agree about max with the exception the COAL used combined with primer brands can vary pressures enough that book max is not max. Then again you can see pressures well below. My 6.5 is 1.5 below and on the edge.

 

[varmintH8R]

I agree about max with the exception the COAL used combined with primer brands can vary pressures enough that book max is not max. Then again you can see pressures well below. My 6.5 is 1.5 below and on the edge.

Totally agree with your entire statement above. As far as what i put in bold, my deal is that lacking a quantitative way to measure pressure, I stop at book max unless I see qualitative pressure signs prior. Not saying my way is the right way, but it gives me a comfort level.

I am afraid of "invisible pressure" if such a thing exists - I have a feeling it does as I have seen Lapua brass eat some loads that I wouldn't be comfortable shooting and look no worse for wear. I doubt it is truly unsafe in modern rifles, but my ignorance of what these limits are keeps me following the book.

How bad are the extractor marks on that factory brass? Any chance you put it on a chrony?

 

[BrentM]

Very faint actually. I only had the two rounds. The kid who owns the rifle shot the rest.

I could measure the head space too. Not sure about this poi brass. Looks decent enough.

 

[woods]

IMO

Primer flattening is a function of headspace and not pressure

Extractor wipe marks can show up on soft brass or some other reason on a load that is safe. Change brass manufacturer

Sticky bolt lift is almost always a good sign of pressure

Ultimate pressure test is primer pockets holding primers on reloads

 

[BrentM]

I would agree except I can take a bran new piece of lapau brass and get the exact same result. For example new brass is 1.832 and hs for my rifle is 1.840. I bump the shoulder back to 1.838. With the exact same load in fire formed and new the primer and extractor mark are very similar.

Accuracy is much better with proper hs though. I was having a newly developed issue which is why I tested all this stuff. Perhaps it is just coincidental.

 

[Kennibear]

Please allow me to interject,

P.O. Ackley said it best, "The absolute pressure of a cartridge is almost solely dependent on the brass case itself. Once the mechanical limit of the brass is exceeded the pressure, no matter what it is, is excessive."

When you see the signs on the brass case itself, the pressure is too much because the brass has exceeded its elastic limit. Doesn't matter what the actual pressure is. Case closed.

Woods: Head space (too much of it) causes flattening. Very true. So does too much pressure. Which one are you looking at? Tough call. I have a Pressure Trace II on 5 guns. When it says the loads are hot the primers are flattened and mushroomed. The primer shank inside the pocket stays at 0.210" but the crown of the mushroom will be about 0.212" for large rifle primers. Cratering can be caused by high pressure or a lousy fit of the firing pin. Look at the bolt face with the pin protruding. Use a magnifying glass. Can you see a gap? Get it fixed! When the primers start cratering trouble is very near...
Another sure sign on the primers is how defined the machine marks of the bolt face are transferred to the primer surface. Magnifying glass again. Use an unfired case and primer to compare. Sometimes the priming punch will lightly mark the primer face too. If the primer looks like a lithograph of the bolt face the pressure is very high, but probably safe. +mushroom +mild cratering and the bolt face markings are signs its too much. I should try to post some micrographs (very close photos) of the primers.

varmintH8R: There is a strange phenomenon seen in pressure traces after the bullet leaves the barrel where the pressure rises again. It has been noted to reach as high as 80kpsi in loads that are safe at 62kpsi at the chamber while the bullet is accelerating down the bore. Why would the pressure rise after the bullet unplugs? Don't know. One load in my son's 300WinMag- Barne's Max with RL19 and the 150gr TTSX- shows a pressure rise of about 8kpsi after the bullet unplugs. Still very safe at about 33kpsi at that point, but why? The strain gauge is centered 1.575" forward of the bolt face so the chamber is seeing this pressure- it's very real.

BrentM: You're astute having spotted the brass problem. Doesn't really matter why except to correct it. That load is a problem so don't shoot any more. Bullet pulling time! My hunch is the brass is screwed up.

But it's still a hunch...