Info on Excessive Pressure Signs for those new to reloading.

Petey308

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Aug 23, 2018
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311
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Missouri
Hello all,

I just wanted to share something a made a while back regarding how to check fir signs of excessive pressure with your fired rounds. I wrote this up to post in other places to help newer reloaders. I figured it might be useful here too.

Signs of Excessive Pressure

Primers are one way to watch for excessive pressure, because they do react to it first and the most, as far as physical changes in appearance. That said, they are notorious for producing false signs of pressure. They're softer than the head of the case, and they're simply friction seated into the pocket on the case head, rather than one solid piece like the actual case. Under 50-70k psi of pressure, the primers will unseat themselves under the pressure of the round firing, the case will expand too, and as everything expands to fill the voids in the chamber, the case head also fills the void against your bolt face. That reseats the primer. The immense pressure will allow the metal of the primer cup to literally flow and that's what results in it flattening as it reseats against the bolt face.

Sometimes you'll even experience cratering, but it doesn't necessarily mean it's from excessive pressure. Remington, for example, actually purposely makes their firing pin holes on R700's larger and puts a slight bevel around it so that you will see cratering first when pressure begins to rise. It doesn't necessarily mean your pressures are actually high or excessive, but it means the pressure is high enough to allow the metal to flow. It's more of a safety function they think helps the inexperienced.

If your hole for your firing pin is larger diameter than your firing pin, you'll experience cratering on your primers. Again, that's due to the metal of the primer cup flowing into the space around the firing pin and surrounding the protruding pin. If it's bad enough, it can flow to the point it thins out around the pin and will allow the pin to pierce the primer (called blanking). This is still not necessarily a sign of excessive pressure. It's more just a sign of a bad fit of your firing pin to your bolt face (the firing pin is just too large).

When the brass of the case itself begins to flow, especially to the point it pushes the lugs of your bolt hard against your action, making the lift of your bolt handle harder (more resistance than opening it with an unfired round), that's when pressure is excessive and you need to back it off a bit. This is also referred to as sticky bolt.

If you wanted to fix the cratering, if you have it, simply get your bolt face bushed to ensure there's no gap/void between your firing pin and firing pin hole in your bolt face.

And yes, some primers are softer than others. That's another reason to not trust primers alone for signs of excessive pressure. Just take note of how the primer looks when you start getting a sticky bolt. If you're trying new primers, take note of each type at that stage. They'll likely all look a bit different. That'll help you later easily reference the primers for changes in pressure caused my things like temps fluctuating.

Look for signs on the case head too. As the metal of the case also begins to flow under pressure, it'll fill any voids it can as well. The hole the ejector plunger recesses into is the most common place the case head flows into. It'll leave a mark on the case head when that happens, referred to as a swipe. When these swipes are pronounced, your pressures are getting high; your bolt handle lift should be experiencing resistance soon, if not already, at that point.

Another way to check for signs of excessive pressure is to take your calipers and measure the diameter of your extractor groove on an unfired case, then measure it again after it's fired and see what the difference is, if any. If you're seeing a big change, that's a sign of excessive pressure.

ARs and other semi-autos are inherently much harder on brass than bolt guns due to the violent auto extraction and how it fires.

As it feeds a round into the chamber, it seats the head of the case in the bolt face. When the hammer strikes the firing pin it hits really hard and with a lot of force. That force not only allows the pin to dent the primer, setting it off, but also allows it to push the case the rest of the way forward in the chamber, seating it hard against the shoulder of the chamber. Then, as the round fires, the pressure forces the case to expand instantly to fill all the gaps in the chamber and back to the bolt face. The immense pressure (even at a safe level) will actually allow the metal in the case to flow and fill these gaps, including into the hole for the ejector. That's what creates those bright marks commonly referred to as swipes. That's also what flattens out the primers. The cratering is the primer flowing around the firing pin and filling the gaps between the firing pin and the firing pin hole in the bolt face. Any bending of the rim is caused from the bolt unlocking right before the pressure stops, causing hard extraction. You can tune this with an adjustable gas block, lighter or heavier BCG, and a lighter or heavier buffer/buffer spring.

Anyways, due to that process, that's unique to ARs and other semi-autos, they produce those false signs of excess pressure. You just have to keep an eye on things. If you blow a primer out upon firing, you definitely know you're over max pressure. And that will occur before you damage the gun, that is if you're working up a load in safe increments.

Measuring the head of your unfired cases, and then re-measuring them after firing will also show you if your pressure is excessive. If the expansion is excessive, that's typically a sign of excessive pressure. So is the primer pocket expanding to the point it will not properly hold a new primer.
 

Mikecr

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Aug 10, 2003
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NC, oceanfront
I'm skeptical of these ideas until separated as problems, pressure problems, and actual excess pressure.
The golden rule is to work up in load. As you do this, you can can see problems, then problems brought on with more pressure, then undeniable excess pressure for the system.

For instance, flattening primers can be expected with fire forming of new cases, because headspacing is high enough to cause it -regardless of load pressure. Cratering around the pin, that's a non-pressure problem. Bush the pin and it stops, regardless of pressure.
Even a pierced primer(unless at a pin crater) is typically a primer cup flaw. If it is caused by pressure, you'll have every sign that exists with it beforehand. Swiping expansion into ejection recess, and expanding extractor grooves are examples of pressure problems. The actual pressure may or may not be too high, but it is excessive for you. There is a difference still.
I don't have ejector recesses in my single shot bolts, so I wouldn't see this swiping, so I could miss an indication here of pressure -or softer brass alloy. But I know that while using soft Norma, I'll see signs before harder Lapua. And I'll see it elsewhere in my testing.
Extractor groove expansion is out of my knowledge/experience base. I don't measure there. Could be a good measure.
There is one thing for sure in all this: if you have excess pressure (that's too high), it's likely you'll see ALL of this, and hopefully it's not too late for you.

But as you work up in pressures, ultimately, you should have well grounded expectations.
A chronograph validates your expectations. Very important tool.
With any new chamber, or new load, I do pressure testing for what I call 'MyMax'.
This is a point where FL sizing would be req'd -of new brass (after a single firing).
What I do is spot measure my case weblines as I go up in load. The webline is a visible highest diameter point at the web area. It's not a standard datum on paper, as case webs vary with their build. Winchester webs for a given cartridge could extend higher or lower than Lapua's webs for the cartridge. The webline will not be a standard diameter to expect either, as our chambers are all different, and our barrels and actions are different.
Anyway, I'll see the webline rise a bit (maybe 1thou) to plateau, then at some point I'll see a step change upward (another 0.0005"). That's MyMax, a runaway condition that has so far correlated with just passing SAAMI max pressure as shown in QuickLoad.
This has been so consistent that I wonder if it matches SAAMI testing method. And of course the chrono has been validating what I'm seeing. I log MyMax, and load development stays below this.

I could have seen problems on the way up to that. If I did, I would stop. I would address it before going further.
It is possible, I see chatter of signs in forums, that your max pressure is lower than others or book.
Every time, this is the result of a bad plan in the build. Usually a poor cartridge chosen for intent, or the wrong chamber or action for it, or mismatched reloading plan.
An example situation that I followed closely, and addressed, is with builds in 26wssm. It has 'short' in it's name so people would build with short actions. But they were missing 'magnum' in it's name. So they would pressure out early, with apparent pressure problems, not because their pressures were high, but because they didn't have enough steel around the chamber for a magnum diameter cartridge.

Unless you pick a wrong powder, it's pretty hard to go over pressure (IMO). But it's easy to create 'problems' otherwise.
 
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