Over pressure signs

Originally Posted By: zillaThere is a great article by James Calhoon on preassure here.. http://www.jamescalhoon.com/



There is an even better article here, by CatShooter. I'm saving this one.
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http://www.predatormastersforums.com/for...nt=1&page=2
 
Back to the primer pocket cratering. Thanks to Cat Shooter I have learned a little more.

What I would like to point out is if you are getting some cratering even at the lowest charge take a good look at the bolt face around the firing pin. I have a Rem. in .243 Win. that showed cratering with the very first shot, ( not a factory round) I thought OH shizz I must have got my load wrong. Went home pulled a couple rds. and all was good. so I thought about the firing pin hole being to big or firing pin being to small. But on investigation I found that the firing pin hole had been chamfered just a little and under a strong magnifying glass I could see that the cratering on the primer was the shape of the chamfered firing pin hole. It is worth the time to take a good look.

DAB
 
Originally Posted By: CatShooterThe first firing in a new chamber is the most critical moment in a cases life. That firing will determine the life of the case.

When a case is fired in a new chamber for the first time, it is a loose fit, and the primer forces it forward and slams it into the chamber's shoulder. At this first firing, there can be as much as 20 thou or more of head space, regardless of what SAAMI says.

When the case is slammed forward, there are two possible outcomes:

1 - when the pressure builds, the case walls expand and grab the chamber walls... then the pressure continues to climb, and the thin section of the case, just in front of the head (called the web) can't hold the pressure anymore, so it stretches - the body is firmly stuck to the chamber walls, and the case head goes back and hits the bolt face. Now we have a case with a thin section just in front of the head - a very badd thing. Now the case is weakened and will never be a good case, even though it may take 4 or 5 firings to fail - it WILL fail, it cannot be fixed.

or

2 - when the pressure builds, the case walls expand and cannot grab the chamber walls.
And, because the chamber and case cannot get a grip on each other, the whole case immediately slides back to the bolt face, and contains the pressure - it does not stretch, and so there is no thin section in front of the web, and it is a strong case forever (or until you screw it up).






How do extractors play a roll here? Or do they?

Push feed vs. controlled round feed.

It seems that the extractor would hold the base of the case to the bolt face [some what]. The controlled round feed especially.???
 
Originally Posted By: DABBack to the primer pocket cratering. Thanks to Cat Shooter I have learned a little more.

What I would like to point out is if you are getting some cratering even at the lowest charge take a good look at the bolt face around the firing pin. I have a Rem. in .243 Win. that showed cratering with the very first shot, ( not a factory round) I thought OH shizz I must have got my load wrong. Went home pulled a couple rds. and all was good. so I thought about the firing pin hole being to big or firing pin being to small. But on investigation I found that the firing pin hole had been chamfered just a little and under a strong magnifying glass I could see that the cratering on the primer was the shape of the chamfered firing pin hole. It is worth the time to take a good look.

DAB

This a very good comment - I have run across several rifles with chamfered firing pin holes - my only thoughts were, "Who the he-ll hired this idiot to make rifles".

Part of the problem is that in modern gun factories, bolts are made on CNC machines, and the drills get feed into the work too fast, so they leave a raggity burr on the face of the bolt. The companies think that chamfering is a quick solution to the problem.

In other industries, this chamfering would be OK. In guns, it is not - it isn't dangerous, but it plays he-ll with readin' primers... and it's just plain piss poor workmanship.

 
Originally Posted By: SmokelessOriginally Posted By: CatShooterThe first firing in a new chamber is the most critical moment in a cases life. That firing will determine the life of the case.

When a case is fired in a new chamber for the first time, it is a loose fit, and the primer forces it forward and slams it into the chamber's shoulder. At this first firing, there can be as much as 20 thou or more of head space, regardless of what SAAMI says.

When the case is slammed forward, there are two possible outcomes:

1 - when the pressure builds, the case walls expand and grab the chamber walls... then the pressure continues to climb, and the thin section of the case, just in front of the head (called the web) can't hold the pressure anymore, so it stretches - the body is firmly stuck to the chamber walls, and the case head goes back and hits the bolt face. Now we have a case with a thin section just in front of the head - a very badd thing. Now the case is weakened and will never be a good case, even though it may take 4 or 5 firings to fail - it WILL fail, it cannot be fixed.

or

2 - when the pressure builds, the case walls expand and cannot grab the chamber walls.
And, because the chamber and case cannot get a grip on each other, the whole case immediately slides back to the bolt face, and contains the pressure - it does not stretch, and so there is no thin section in front of the web, and it is a strong case forever (or until you screw it up).






How do extractors play a roll here? Or do they?

Push feed vs. controlled round feed.

It seems that the extractor would hold the base of the case to the bolt face [some what]. The controlled round feed especially.???




No!... The extractor can't hold the case against the bolt face.


Take your bolt out, and take a case and hook it under the extractor - now wobble it around and you will see there is HUGE play.

The primer is like a "Ram-set" cartridge - when it goes off, it really slams the case forwards - Richard Diaz, head of the Remington Research Labs, told me that the primer force is about 700 pounds (NOT pounds per square inch, but real pounds)... it makes sense to me, as I have a stainless steel pot with a deep dent in the bottom - a primed case went off in it (don't ask
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) and the case stuck in the ceiling and the primer made a large dent in the pot.


.
 
How much different are gas guns with the headspace/ primer flattening were full length resize dies are used. Iv noticed minor bolt swipes and the primers still dont look too bad.
 
Originally Posted By: Widow maker 223How much different are gas guns with the headspace/ primer flattening were full length resize dies are used. Iv noticed minor bolt swipes and the primers still don't look too bad.

Gas guns are rough on cases... they need the cases to be loose enough to easily feed, which means that the cases need to be smaller than the chambers, so they expand with each firing, and you will get a separation eventually. With very careful case prep, you can stretch it, but in the end, the effort might not be worth it.

As to wipe, the bolt starts to rotate before pressure is "0". The pressure is low, but often enough to cause wipe marks on the case, even though the pressure is within limits.
 
Originally Posted By: venaticOriginally Posted By: MPFDThis outta be a sticky!

Considering doing that.

Sounds like a great idea. I've already printed Catshooters post on the die setup, linked it to other posts, and emailed it to a few buddies. Good stuff in here, might as well make it easy to find.
 
Originally Posted By: Widow maker 223How do you know when u have reached high pressure in a gas gun?? Is the primer flattening not a big deal the same as with a bolt gun? Good question. This is a question I have as well. I often get mild primer flattening with min to mid loads in the AR. If I don't see any cratering or other pressure signs on the brass I ignore it and keep going, but I'm not sure if I'm right in doing that.
 
If you're lucky, this is all that will happen when you spark off an overpressure load in an AR...
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P-doggin09028.jpg


That's what is left of a .20TAC case after beating the bolt open with a cleaning rod & a 2x4. The rest of the case was still stuck in the chamber
frown.gif


That was a warm load, turned hot in the Summer Sun. Combine that with a long string of p-dog fire and rounds baking in the chamber and KA bLEWWWeeee!!! I went backwards off the stool...

I keep that tuliped mag frame on my reloading bench to remind me of that day!!!
 
/\. That's why i like temp insensitive powders.
My mild loads have rounded edges in my AR. But the hotter loads have mild swipes and flattened primers, but no cratering.
 
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Fred what powder was that? IIRC you at one time were using H4198 in your .20Tac. If so it was a supposed temp insensitive powder.
 
Originally Posted By: Widow maker 223How do you know when u have reached high pressure in a gas gun?? Is the primer flattening not a big deal the same as with a bolt gun?

"Is the primer flattening not a big deal" - Not a big deal.

Cratered primers and head smearing are the two signs to look for (plus any malfunction).

If you are not very experienced in reading cases and primers, you can NEVER go wrong by looking at a fired factory case from your gun, and being cautious when the cases/primers look different.
 
Originally Posted By: GrizleyHunterCatShooter,
Please explain in more detail what wipe marks are and where they appear.

Wipe marks look exactly like that...

Think of taking a piece of metal, placing it flat ont he case head, then rotating it under pressure - the case head gets scratches going around the case face like you ground into something - if you get them you CAN'T mistake them.

Same with ejector hole marks - looks like a piece of brass was sticking out of the case head, and then was cut off (which is exactly what happens).
 
Originally Posted By: knockemdownIf you're lucky, this is all that will happen when you spark off an overpressure load in an AR...
scared.gif

P-doggin09028.jpg


That's what is left of a .20TAC case after beating the bolt open with a cleaning rod & a 2x4. The rest of the case was still stuck in the chamber
frown.gif


That was a warm load, turned hot in the Summer Sun. Combine that with a long string of p-dog fire and rounds baking in the chamber and KA bLEWWWeeee!!! I went backwards off the stool...

I keep that tuliped mag frame on my reloading bench to remind me of that day!!!

While it was obviously excessive pressure (a BUNCH) that took that apart, it wasn't an "excessive pressure round" - a normal pressure round will do this when subjected to sitting in a hot chamber for long periods of time.

This is what is called a "Cooked round" - similar to a cook-off, except you shoot it before it shoots itself.

When a round sits in a hot chamber and the temperature raises tremendously, it is a big problem - no powder is stable enough to withstand sitting in a hot chamber for a long time. It's one of the disadvantages of shooting an AR over a Dog town.
 
Another big THANK YOU for taking the time to post all the great info.I think you have answered two things I've been wondering about as I am a new reloader.
Primers flattening on factory hornady loads and some lapua cases not needing to be trimmed.
Not sure if the hornady loads are primer or shoulder related but it is a moot point as I don't use them anymore.I forgot to clean my 1st batch of lapua cases after sizing and suspect those are the ones that aren't growing.Thanks for the primer!
 
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