BRASS CASE EXPANSION

[marine one]

Hello everyone,
I have a question that I'm sure everyone has came across.

In reference to brass cases shanking:
After loading and firing a round I measured the brass case and it was smaller then it was before.
I reloaded same round..The caliper is 7.62x39, the case measured at 1.518 before I added powder
and bullet.After firing I measured case 1.512.
I thought when you fire around the case gets larger. My rifle is a CZ 527.
If anyone could share some info regarding this situation, it would be greatly appreciated.

 

[cfullgraf]

When fired, the case expands to fill the chamber then shrinks back some. But, it does not return to the original size it was before firing.

Hence, the overall length gets shorter because the body of the case is a larger diameter than before firing and it draws the case mouth towards the base of the case.

When resizing the case, the body diameter gets reduced and the brass has to go somewhere so the case gets longer.

Within this process, there is some plastic flow of the case walls, they get thinner and the now excess brass has to go somewhere so the case length grows past the original case length.

Most of these changes cannot be measured except for case length with the tools most reloaders use.

Hope this makes sense.

Look up plastic and elastic flow of metals on the internet.

 

[deadeye dick]

Very well stated. That's exactly what happens.

 

[marine one]

Can I reload that case if it fits in my chamber and I don't resize?

 

[.308 Norma]

Can I reload that case if it fits in my chamber and I don't resize?

Not exactly. Even if the case fits in your chamber, you'll have to resize its neck in order to get it to hold the bullet. There's a lot of problems that can arise from having insufficient "neck tension" - the case not having a good enough "grip" on the bullet.
The case neck is the only thing I resize when I reload some types of bottlenecked rifle cartridges. The cases still fit my rifle's chambers, so I save wear and tear on them by not resizing them completely, which sets their shoulders back. Some folks claim better accuracy (precision) too with "neck-sized only" cases. I'm not a good enough shot to tell any difference.

 

[cfullgraf]

Can I reload that case if it fits in my chamber and I don't resize?

To add what .308 Norma said, if you are shooting a semi-auto rifle, you have to full length size the case otherwise, it will not chamber. Lever action and pump action rifles probably should also be full length sized due to the weaker chambering and extraction process of the action.

Also, generally, bottle neck cases that are only neck sized will only re-chamber in the rifle they were fired in and you may need to full length size them every few firings.

Like .308 Norma, I do not see any difference in accuracy between neck sized cases and full length sized cases as I am also not a good enough shot. I do neck size my 22 Hornet cases and 7mm International Rimmed cases to extend case life but I only have one firearm each that fires those cartridges.

 

[Bcwitt]

If you had a piece of metal tight in a vise, and you heat it w a torch, it will expand in all directions to the limit of the vise, then it will expand in every other direction (un restrained). When cooled, it contracts uniformly inward. It will then be looser in the vice (and permanently shorter) in the restrained area. I think your case is behaving like that.

 

[marine one]

OK but at what length do you keep your brass at? if the trim length is 1.518 max at1.528, on a 7.63x39 cartridge.After firing,and full length resizing
and sizing my neck down I still come up shorter then 1.518. I come up with 1.514,1.513??

 

[cfullgraf]

Do you know what the length of the case is new, before any firings? They could be short to begin with.

As long as you have adequate neck tension when the bullet seated, a few thousandths short is not an issue. The case will “grow” over the next several firings and sizing cycles.

I trim my cases when they are a couple thousandths short of the max listed trim length and then trim them to a couple thousandths longer than the minimum length.

Other folks have their own limits for trimming.

 

[marine one]

Thanks cfullgraf, I like your logic and how you trim your brass, are you firing them in a bolt action rifle?

 

[Slamfire]

You are shooting a bolt gun, and what measurements you are taking will be different if you had shot the case in a gas gun.

With either action, the bullet is out of the barrel and pressure is zero before your finger has left the trigger. And with a bolt gun, the action is locked after pressure is zero. However, with gas guns, they are deliberately timed to open up while there still is pressure in the barrel. The pressure is below the rupture strength of the brass case, but it is enough to keep the front of the case stuck to the chamber.

This is a pressure curve of a gas gun. It happens to be a modified Garand receiver with a 308 Garand barrel, with a standard Garand gas cylinder and operating rod.



as you can see, unlock is occurring when there is still residual pressure, and this is deliberate. The intent is to increase the time that useful gas pressure is available. And to help pop the case out of the chamber. In Col Chin's Machine Gun series, this is called the residual blowback affect. During this period the front of the case is still stuck to the front of the chamber, and as the bolt goes back, the case elongates. And it is measurable.

The headspace on the M1a these rounds were fired from was cut by Springfield Armory to 1.630". Upon ejection, the cases were all longer than the longest allowable, which is "No Go" on my cartridge headspace gauge.



I highly recommend using cartridge case headspace gauges in setting up your dies,





and sizing to gauge minimum,.



I have found that neck sizing will only lead to extraction problems in time, and use of neck sized ammunition in a different gun, will lead to extraction problems sooner. Also, no one I know who has won a National Long Range Championship ever neck sized their ammunition. These guys want accurate, and reliable ammunition. And having to break position, then find a cleaning rod to knock a case out of a chamber, is a sure recipe to be a loser in a timed event, where the wind changes in a moment.

 

[cfullgraf]

Thanks cfullgraf, I like your logic and how you trim your brass, are you firing them in a bolt action rifle?

I have both but for the most part I've found cases fired in a bolt action gun grow slower than those fired in a semi-auto rifle.

In part, this assumes the sizing die has been set up correctly and does not move the shoulder back too far during sizing.

With 204 Ruger and 223 Remington, I have bolt and semi-auto actions in both cartridges. Frequently, after a few firings in the semi-auto rifle, I move the cases to the bolt action rifle to extend the case life a bit.

 

[marine one]

Thanks to all who responded,Iv'e learned a few things In my old age. It's good to belong to this forum.