The importance of case trim

[eschafer]

""I have a chamber cast made from when I bought the rifle. The same place ahead of the belt on the chamber cast measure 0.511, the fired case measures 0.516 .""

without hijacking this thread: but how is it possible for this to happen? I understand the cast's measurements change with time, but I've heard of this before... how does it measure just above the belt, larger than the chamber.?.. an answer to this could gell my understanding of what actually happens to a case when fired... thanks

 

[Edd]

I don't think he said he had case head separation either. He did say there was a place on the case that was .015" larger than the chamber. I'm still stuck on that.

 

[sab]

eschafer and Edd:

Brass is an elastic material, as is the steel making up the chamber, but only to a certain point. After that, they become plastic (permanently expand). When pressures are too high, the high pressures inside the case will expand the case and the chamber beyond their sizes with just atmospheric pressure in the chamber. Because the brass transitions from elastic deformation (able to "spring back") to plastic deformation (permanently stretched) at a lower pressure, as the pressure subsides, the chamber springs back to it's original, smaller diameter, but the case only springs back so far, leaving it larger after firing. This phenomenon is what causes a stuck case. Together, the case and chamber expanded under pressure, and when the pressure dropped, the case was left larger than the chamber, but the chamber is much stronger, so it compressed the case elastically, causing the case to be stuck in the chamber. Once removed, the case springs back to the diameter that is larger than the chamber.

I hope this makes sense. It's a complicated situation, and I remember sitting in my materials science class 30+ years ago and thinking about a rifle firing when the professor was explaining a stress-strain curve to us. Today, a student having those thoughts best not share them, or they'll be suspended for even thinking about guns...

SAB

 

[Mikecr]

A longer case means the entire case OAL is longer. It does not mean the neck itself is longer. It says nothing of shoulder bump where headspacing off shoulders.
Sinclair sells a chamber end gauge and this is simple & useful for determining clearance from case mouths -provided shoulder fitting(your datum point) is known/controlled.

This is basic stuff reloaders should be engaged in before even thinking about trimming brass away.

 

[Mikecr]

The interference fit sab describes is due to lack of breech support for pressure applied, and excess chamber clearances, both allowing plastic deformations.
It doesn't always mean pressure is excessive.

 

[ntsqd]

SAB,
Was thinking the same explanation, but a .015" delta seems like a lot.

If the case OAL is measured and found to be longer, that is all that we know at this point. We don't know where it grew, only that it did. Could be the shoulder moved, could be the neck only lengthened. We just don't know from that one measurement. So let's go easy until we know more.

 

[Gary kittelson]

That's Great, But the fact that he had a case head separation stills bothers me. Case head separations are normally a sign of excessive head space that allows the case body to move forward before ignition and grip the chamber walls during ignition and the case head is forced back to contact the bolt face, stretching the case in the web.

A case that Is to long, prevents this from happening, But raises the pressure because it does not/cannot release the bullet correctly.
lowering the pressure will not change the head space and case head separation can still happen over time.

If head space is correct and the chamber is correct and case head separation still occurs, the load was extremely over pressure and is setting the bolt lugs back or stretching the action. Ether one is not good.

Just trying to warn the op that there may be other problems.

J E CUSTOM

I often disagree posts by others, and always agree with you. Are we always right or always wrong?

 

[eschafer]

eschafer and Edd:

Brass is an elastic material, as is the steel making up the chamber, but only to a certain point. After that, they become plastic (permanently expand). When pressures are too high, the high pressures inside the case will expand the case and the chamber beyond their sizes with just atmospheric pressure in the chamber. Because the brass transitions from elastic deformation (able to "spring back") to plastic deformation (permanently stretched) at a lower pressure, as the pressure subsides, the chamber springs back to it's original, smaller diameter, but the case only springs back so far, leaving it larger after firing. This phenomenon is what causes a stuck case. Together, the case and chamber expanded under pressure, and when the pressure dropped, the case was left larger than the chamber, but the chamber is much stronger, so it compressed the case elastically, causing the case to be stuck in the chamber. Once removed, the case springs back to the diameter that is larger than the chamber.

I hope this makes sense. It's a complicated situation, and I remember sitting in my materials science class 30+ years ago and thinking about a rifle firing when the professor was explaining a stress-strain curve to us. Today, a student having those thoughts best not share them, or they'll be suspended for even thinking about guns...

SAB

makes perfect sense..... thank you for the very concise explanation.....

 

[eschafer]

The interference fit sab describes is due to lack of breech support for pressure applied, and excess chamber clearances, both allowing plastic deformations.
It doesn't always mean pressure is excessive.

and this too makes perfect sense.... I keep thinking of the dreaded "bulge above the belt" (so often argued about on many sites)... I've seen it with my 338 wm... because support at breech is less than remaining chamber, breech is more likely to expand when fired. therefore, the "bulge above the belt" on a case happens when fired and NOT from sizing... got it now... thanks fellows....

 

[sab]

SAB,
Was thinking the same explanation, but a .015" delta seems like a lot.

ntsqd:

I don't see where he mentioned a .015" expansion. Below, he mentions .005", which is within the realm of possibility (ignoring the fact that chamber-casting alloys like Cerrosafe initially shrink, then expand, so the measurement is best done an hour after casting):

I have a chamber cast made from when I bought the rifle. The same place ahead of the belt on the chamber cast measure 0.511, the fired case measures 0.516

 

[ntsqd]

My bad, I saw the .015" number above and went with it. Seeing the .005" number it seems much more reasonable.

 

[Edd]

In the original post, he said it expanded .020".

 

[J E Custom]

I often disagree posts by others, and always agree with you. Are we always right or always wrong?

Hopefully, Most of the time we are right.
Most if not all of my post are based on experience over many years and repairing/re barreling many rifles with problems. Opinions vary based on different experiences so I'm not surprised if someone disagrees. But It may not change my mind or way of doing something.

J E CUSTOM

 

[John Porter]

eschafer's explanation is very useful to remember. A quick and accurate [in my experience] way to make a cast of your chamber is to full length resize a case and fire a "reduced load" with a 95% full case of ADI Trail Boss Powder [IMR Trail Boss in the U.S.A] and any medium weight projectile. You will notice that the discharge is quite "low energy", but it is enough to fully fire form the brass to your chamber without distortion. I once did this with three different makes of brass in a competition 308 chamber and all three came out with not even a thou between them.