Home Forums Hunting & Shooting Ask The Gunwriters Pressure signs.

How much over pressure does a cartridge need to be to show pressure signs? Now that I've got quick load I'm questioning a lot of my loads.

As a very rough rule of thumb, pressure signs start around 70 KPSI and primers fall out around 80 KPSI.

If you wake up behind the bench with a scope stuck in your forehead, chances are you should drop down at least a grain or two.

Once I started actually conducting pressure testing, I began to question my "youthful load development".

One day we pressure tested some proof loads that were ~80,000 PSI.

Visual observation of case #1 would make most handloaders say "Think we are just about maxed out. Might stop there"

Visual observation of case #2 would make most handloaders say "Wow, we are getting great velocity but not much pressure sign. Let's go up another few tenths and see what happens".

I went home and pulled some handloaded cartridges that evening.

I watch velocity closely now (don't have pressure test equipment any more). If the velocity is high, I know the pressure is high.

Agree.

Here's an example of primers falling out over 80K.

I've posted this before. I'll post it again as a reminder that even a "careful" reloader can make a mistake.

https://www.24hourcampfire.com/ubbth.../Number/9781434/Searchpage/9/Main/680006

DF

Chronograph.

Chronograph.

Chronograph.





Dave

mog75,

One of the classic examples of what Denton posted is the 7mm STW. When it was a wildcat, most fans reported 140-grain bullets getting 3600-3700 at safe pressures and 160's at 3400+. Safe pressures meant no "pressure signs."

When Remington started pressure-testing the more popular handloads the vast majority were between 70,000 and 75,000 PSI. Which is why Remington's 140-grain factory loads stopped at 3400 fps.

The strength of the brass affects the safety of loads as well. I've had book starting loads of Varget loosen primers on Federal .308 brass. The loads were pulled & double checked, and the charges & powder were correct. I've also seen Federal 7mm-08 brass that was a little soft, and .38-55 Starline brass that started to loosen primers before they should have. In situations like that, I look for stronger brass, if available.

In general, if I have a rifle that is very strong compared to the brass, I will watch both the chrono and the case heads. Short brass life implies the load is too hot, regardless of action strength.

Excessive headspace can also cause flat primers, even if the load is not excessive.

If the action is old or a less strong design, then I'm going to watch the chrono and use book loads, to avoid stressing the action.

Fooling around with a .300 RUM, I've run some loads that were quite fast & pretty accurate, with good case life in strong Nosler brass, but in the end I backed down some and found even better accuracy.

measuring case head expansion on virgin brass ?

I have found the chrono to be invaluable, if the speed is really great.. Perhaps I should look a bit further. To me an absolute indication is a flattened primer, ejector marks on the base (depending on the action) and a sticky bolt lift.

Quick load so far has been extremely close on the velocities(never more than 50 fps off), but the pressures are sometimes extremely high. Can I assume that when it's nailing the velocities, that it is also pretty close on pressure?

ldholton,

Measuring CHE doesn't work UNLESS you have a pressure-tested load to compare it to, using exactly the same brass. I've run several experiments on this in a commercial piezo-electronic lab, and brass varies so much in thickness and the exact composition that anybody who claims X amount of expansion means Y PSI doesn't know what they're talking about, much less whether the load is "safe."

A chronograph is a far better indicator than CHE.

That's kind of what I was thinking. no real control suspect to start from

Thank you all very much.

If you shoot a lot you can detect over-pressure by recoil alone.

To be clear, my point about checking CHE is that if the solid web of the case is expanding, the load is too hot. Not talking about the pressure ring, ahead of the web, which does expand and get resized by a FL sizer die. The cause might be one of several things; it could be soft brass. I use it more as a double check, while also looking at the chrono results.

I too was referring to that. Always read that .0001 was or .0002 was max and .0003 was hot. but wondered about how to tell how much certain brass would expand at a known pressure.

It would have to depend on the brass, not only its yield strength, but also the case design.

Impressive - a calibrated shoulder!!

Regarding head expansion or pressure ring expansion measurements...would the following method have a decent degree of accuracy?

Fire several rounds of factory ammunition and measure.

From the same box of ammo, pull bullets, dump powder, and remove primers from several rounds.

Replace primers with what you want to use; do the same with powder and bullets.

Fire several rounds, record measurements and compare with initial measurements of factory ammo. Of course, chronograph everything.

This is assuming that the brass in one box is all from the same batch and has identical metallurgy. Is that a reasonable assumption?