Primer Signs & Pressure

[AkMtnRunner]

I am working up a load and would like to hear some thoughts on pressure. This is a 7 PRC, 160 LRX, Fed 215GM primer, Hodgdon 4831SC, new Peterson brass. The barrel is brand new by Patriot Valley Arms, 21" and 1:7" twist. I am just starting out the load work up, just trying to find where the pressure limit is.

Below are a few photos of the primers. Back row - left is lowest charge going right, then front row rightmost is the highest charge. Disregard the front-most row partially out of frame. The lowest charge weight is 62.0 grains, all the way up to 68.0 grains. Barnes' starting load is 59.6 and max is 66.2. I should note that I am using a little longer COAL (3.38") than Barnes' (3.325"). I did load 6 rounds at Barnes' recommended COAL, which velocities were very similar to the same charge weights of the rounds with the longer COALs.

Despite exceeding book max charges, bolt lifts were easy all the way up to the highest charge weights, no extractor marks, and velocities were not impressive (~200 fps less than book expected with just 3" more barrel length).

To me, the primers do not look too flattened but I can certainly see a trend becoming more flat with higher charges. I understand a longer COAL can decrease pressure, so that was the reason why I tried some at Barnes' recommended COAL, but that turned out to not make any significant difference. I've measured the COAL for this bullet to engage the lands at 3.52" in my barrel.

What say ye? Safe to keep going up in charge or just accept this a slow barrel, perhaps due to excessive twist?

 

[Axlrod]

The very best indicator of over pressure is heavy bolt lift. Everything else is more subjective and less accurate.

 

[SDHNTR]

COAL is not going to change pressure any appreciable amount until you are jamming into the lands. And you’d need to show us better close-up pictures of your case heads to see any signs of over pressure.

 

[AkMtnRunner]

Here's a closer photo of the highest charge.

 

[Ckat]

I read virtually nothing into primers beyond them being pierced or blown. I look for heavy bolt lift and ejector swipes.

 

[SDHNTR]

I see nothing alarming

 

[35WhelenAI]

I see nothing alarming

I do, he exceeded max book charges by almost two grains and is considering going higher yet. Without measuring case head expansion you know nothing.

 

[parshal]

If you really want to know if any of those are over pressure, spritz a little water on the case and fire it. If you get any change to bolt lift it's over pressure. You would be surprised how many folks' "safe" load will show signs when the case has a little water on it.

This test simulates a snow or rain on cases.

 

[parshal]

Here's the load before water:



And here it is with a little spritz of water:



He didn't believe me when I said it was over pressure just based on velocity alone. I don't remember the velocity difference when the case was spritzed but there was heavier bolt lift along with ejector marks.

 

[AkMtnRunner]

I do, he exceeded max book charges by almost two grains and is considering going higher yet. Without measuring case head expansion you know nothing.

Just measured expansion from case head to shoulder. The cases of charges below book max of 66.2 are consistently 0.005” longer than an average of new brass while the ones at book max and above are all 0.006”. I am not sure how significant that is.

Thanks to all lending your ideas. I will say that I am not necessarily wanting to go higher, rather I just want to find where the safe pressure limit is.

 

[Grunt-N-Gobble]

What velocity were you getting @ book max? Barnes loading data tends to have max charges higher than other data I've found for the same bullet so be cautious.

I'm working on load development for my 7PRC same as you. However while I have H4831sc, I'm starting with H1000 using the 160LRX.

 

[35WhelenAI]

Case head expansion is measured around the diameter of the case head just up from the extractor groove with a dial micrometer. Has nothing to do with fired case length unfortunately. That has to do with case stretch, that's the brass simply expanding at the thin case walls to fill the chamber and make the seal. It won't go past a certain point because the chamber only goes so far forward. Which is why there's not a lot of difference between light loads and book max loads.

In a nutshell, fire minimum charge reloads and measure the case head at the proper place. Then fire reloads as you increase the charge. 0004"-0006" expansion is considered max load.

Do a search for it here or on the internet, you will see what I'm talking about. I've posted the method in other threads on this forum over the years.

 

[AkMtnRunner]

What velocity were you getting @ book max? Barnes loading data tends to have max charges higher than other data I've found for the same bullet so be cautious.

In the original post, I noted the velocity at book max is ~200 fps slower than expected book velocity. This is a 21" barrel vs Barnes' 24" test barrel.

Case head expansion is measured around the diameter of the case head just up from the extractor groove with a dial micrometer. Has nothing to do with fired case length unfortunately. That has to do with case stretch, that's the brass simply expanding at the thin case walls to fill the chamber and make the seal. It won't go past a certain point because the chamber only goes so far forward. Which is why there's not a lot of difference between light loads and book max loads.

In a nutshell, fire minimum charge reloads and measure the case head at the proper place. Then fire reloads as you increase the charge. 0004"-0006" expansion is considered max load.

Do a search for it here or on the internet, you will see what I'm talking about. I've posted the method in other threads on this forum over the years.

Thanks for the clarification. You have .0004 to 0006" . . . should that be 0.004-0.006", as in 4 to 6 thousandths? I've measured that diameter expansion to between 0.0045" and 0.0055". The strange thing is there almost no correlation between charge weight to expansion. From lowish to exceeding book max, there doesn't appear to be a trend to greater expansion.


I've never reached book max charges without seeing the signs of high pressure until this one. Whatever, I am thinking I'll just use sub-book-max loads for the OCW testing. I was debating between 6.5 and 7 PRC for this rifle so I'll be okay if it's not shooting at the upper end of what the 7 can do.

 

[mtnbound]

Case head expansion is measured around the diameter of the case head just up from the extractor groove with a dial micrometer. Has nothing to do with fired case length unfortunately. That has to do with case stretch, that's the brass simply expanding at the thin case walls to fill the chamber and make the seal. It won't go past a certain point because the chamber only goes so far forward. Which is why there's not a lot of difference between light loads and book max loads.

In a nutshell, fire minimum charge reloads and measure the case head at the proper place. Then fire reloads as you increase the charge. 0004"-0006" expansion is considered max load.

Do a search for it here or on the internet, you will see what I'm talking about. I've posted the method in other threads on this forum over the years.

I would think that brass hardness or strength, along with chamber variations, introduces a significant variable when using CHE as a method to examine over-pressure loads. I assume you could only use the measurements from the same brass lot and chamber. Then, whenever you would change, you would need to develop a new baseline.

 

[mtnbound]

Here's the load before water:

View attachment 890050

And here it is with a little spritz of water:

View attachment 890051

He didn't believe me when I said it was over pressure just based on velocity alone. I don't remember the velocity difference when the case was spritzed but there was heavier bolt lift along with ejector marks.

So is the spritz of water cooling the brass or filling space in the chamber?

 

[S.Clancy]

With primers I just check to see if there's a raised edge around the firing pin indent. If I can scrape a finger nail across the firing pin indent and not catch, it's fine. If there's a little ridge I back off.

 

[35WhelenAI]

In the original post, I noted the velocity at book max is ~200 fps slower than expected book velocity. This is a 21" barrel vs Barnes' 24" test barrel.






Thanks for the clarification. You have .0004 to 0006" . . . should that be 0.004-0.006", as in 4 to 6 thousandths? I've measured that diameter expansion to between 0.0045" and 0.0055". The strange thing is there almost no correlation between charge weight to expansion. From lowish to exceeding book max, there doesn't appear to be a trend to greater expansion.


I've never reached book max charges without seeing the signs of high pressure until this one. Whatever, I am thinking I'll just use sub-book-max loads for the OCW testing. I was debating between 6.5 and 7 PRC for this rifle so I'll be okay if it's not shooting at the upper end of what the 7 can do.

Negative. A dial micrometer to 4 decimal places. .0004-.0006. Your measurements are quite a magnitude greater than what you should be measuring. 45/1000-55/1000 is what you are calling your measurement, I'm talking for 4/10000-6/10000. If you are measuring that much head expansion it's either a blade micrometer which is not capable of the accuracy or way overboard. You must use a dial micrometer. Different than what you typically use for reloading.

 

[SDHNTR]

Just measured expansion from case head to shoulder. The cases of charges below book max of 66.2 are consistently 0.005” longer than an average of new brass while the ones at book max and above are all 0.006”. I am not sure how significant that is.

Thanks to all lending your ideas. I will say that I am not necessarily wanting to go higher, rather I just want to find where the safe pressure limit is.

That’s not what he was referring to. Case head expansion is measured as a diameter just above the rim.

 

[35WhelenAI]

Here's the search for case head expansion that took just a few seconds to pull up. Read up what I posted. It takes patience and precision, you can't half-ass it together.

P

Post in thread 'General Reloading Advice - Pressure and Velocity'

Dec 29, 2024

Case head expansion measured with a micrometer in .0001”, or the number of times a case can be fired before a partial head separation starts to form or primer pocket is loose, are reliable means of comparing loads and determining max. If a case only lasts less than 5 firings the load is too hot, if the case lasts 8 to 10 firings that’s a safe max, regardless if velocity matches something printed in a book.

Sticky bolt lift may be pressure, or may be straight walled case in a rough chamber.

The more pieces of evidence a person can get on the load the better.

So if you...

• Packmansion

 

[BBob]

That’s not what he was referring to. Case head expansion is measured as a diameter just above the rim.

And to do it properly you pretty much need to use a blade micrometer that'll measure to the tenth (.0001"). It's still not the best indicator due to varying brass hardness between brands of cases.

Those max loads have pretty flat primers and heavy cratering. I'd say you are very close to max pressure.