Lets talk under pressure signs....

S

std7mag

New member
Ok, most everything on the web, and in the books is on overpressure.

But what do you look for with underpressure?? (aka the short load)

Give us your thoughts please....
 
Sooty case necks and even sooty down 1/2 way on the case. Usually this soot can be simply wiped away. Cases don't expand enough to seal the chamber. You can even get sunken spots on the case shoulder from low pressure. Some sooty necks are the result of multi-fired cases that have work hardened and need annealing rather than low pressure. Primers backing out of the pocket and staying high is an obvious sign. Primers back out of the pocket against the bolt face but the case does not expand enough to push the primer back flush into the pocket. Most common reason for low pressure is low powder charges and another reason is using too slow of powder for the cartridge you're loading. Very poor bullet tension can effect pressure. Unburnt powder can be a sign. Another good reason to use a chronograph too checking for lower than normal velocity.
 
Bullet stuck in barrel. ;)

Sooty cases.
Gas blow-back.
Erratic ignition.
Erratic velocities.
Poor performance.
Short headspace after firing (with bottleneck cartridges).
Unburned powder left in the barrel (and/or case and chamber).
Improper function in auto-loaders.
Unstable projectiles from velocities too low for the twist rate.
Primer back-out, as mentioned by rg1.

And, I'm sure we can come up with two dozen more.
 
It is possible to render a pistol scrap with reduce loads. Problem: About the time a reloader/shooter thinks it can not happen, it happens. Another problem, it does not happen every time and is not repeatable.

F. Guffey
 
Thank you all for your responses...

Reason I brought it up is that I chambered a rifle to 7mm-08 Improved with a 40 degree shoulder. Load data is essentially non existent.
I'm thinking I'm actually getting low pressure, not high pressure signs.
Tons of soot, as was mentioned.
Rims of casings are actually widening, making extraction difficult, and causing much brass to be thrown in the scrap.

Have started my loads about middle of load chart, and worked up from there in .3gr increments. Have usually been stopping about max load for standard 7mm-08 in an attempt at self preservation. (aka I don't wanna blow anything up)...
 
When loading for mosin I got some funny low pressure signs.

I was using 150 gr bullets. The powder was either varget or n135. I loaded light as it was during the dry year and I was down to my last half pound. The casing was sooty as expected. Some brass even had partially collapsed shoulders. To this day I still can wrap my head around how it happened.

When I switched to 174 gr bullets, the brass was still sooty, but no more collapsed shoulders.

-TL
 
The casing was sooty as expected. Some brass even had partially collapsed shoulders. To this day I still can wrap my head around how it happened.
Click to expand...

I could say simple but etc..

Your pressure was too low, with more pressure your case would have expanded and sealed the chamber. But with less pressure your neck did not expand, because the neck did not expand pressure blew gas back between the case body and chamber.

And then when the pressure inside the case dropped the pressure trapped between the case body and chamber crushed the case. When I form cases I do it once, I have been accused of 'getting into some risky stuff'.

F. Guffey
 
velocity on the 7mm-08 improved is comparable to the .280 rem original.however the 7mm-08 is a wider shell and that benifits velocity so i dont know if you really want to load to .280 rem specs.

people always say you know,you can reduce the charge of a .308 load by 10% and thats a good .300 savage load but this 7-08 im. is in unchartered waters.

Jerrett chambers for the 7mm-08 improved,call them on the phone ask if you buy some of there brass or maybe even some of there ammo.see if they will give you the specs for 7mm-08 improved
 
F. Guffey said:
I could say simple but etc..

Your pressure was too low, with more pressure your case would have expanded and sealed the chamber. But with less pressure your neck did not expand, because the neck did not expand pressure blew gas back between the case body and chamber.

And then when the pressure inside the case dropped the pressure trapped between the case body and chamber crushed the case. When I form cases I do it once, I have been accused of 'getting into some risky stuff'.

F. Guffey
Click to expand...
What I can't understand is why the pressure outside the brass could be higher than inside the brass. When the pressure "blew back" should it get inside the brass as well as between the brass and the chamber wall? For reasons beyond my comprehension, the pressure chose get between the narrow gap between the brass and chamber, instead of the wide space inside the casing.

-TL

Edit: I think I found the answer. The key is volume. Insider the brass is big. The volume between brass the chamber is small, and the gap is narrowing. It is the gas law. For the right combination, the pressure outside could be higher than inside. It is more likely when the brass is about to seal. It actually got worse when I increased the powder charge slightly. That was why.
 
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Edit: I think I found the answer. The key is volume. Insider the brass is big. The volume between brass the chamber is small, and the gap is narrowing. It is the gas law. For the right combination, the pressure outside could be higher than inside. It is more likely when the brass is about to seal. It actually got worse when I increased the powder charge slightly. That was why.
Click to expand...

Not really, pressure was equal on both sides of the case, the pressure dropped fast on the inside of the case, pressure on the outside of the case escaped much slower, and that is the reason the case crushed.

Again, the pressure was trapped and could not escaped until it crushed the case.

F. Guffey
 
F. Guffey said:
Not really, pressure was equal on both sides of the case, the pressure dropped fast on the inside of the case, pressure on the outside of the case escaped much slower, and that is the reason the case crushed.

Again, the pressure was trapped and could not escaped until it crushed the case.

F. Guffey
Click to expand...
It is the same thing. Just different ways to describe it. Thanks for getting me rethinking the whole thing.

-TL
 
What about rim flattening???

I was wondering why the bolt was hard to open, and hence stopped my load testing there.

I was then having issues with closing, and opening the bolt the next time I reloaded. I took the bolt apart, and inspected, along with a bore scope at the chamber. Couldn't find anything wrong.

When I tried to put the casing in a shell holder for trimming, is when I found the rim issue. Have since discarded those casings. But would still like to know, ya know...
 
I haven't heard of rim flattening. So the rim's diameter just gets bigger?

The rim is outside the chamber. Even it gets bigger, it should not cause difficulty to extract. Rifles with closed breech face, such as rem 700, may have problem ejecting or closing the bolt. But shouldn't be extraction.

I don't see why and how light loads flatten rims.

-TL
 
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