How does brass extrude?

[762 shooter]

When you trim a piece of fired brass, does the brass come from the base, the neck, or the full length? Is head separation this thinning of the brass? Why would it separate at the thickest part of the brass?

I am loading some 308 cast boolits into the same brass quite a few times with no signs of failure, (light loads).

Just wondering.

762

[btroj]

The brass does come from the head area of the case. Tis is why some cases develop a head separation

Think of it this way. A case has a few you of wiggle room fore and aft when chambered. Case is driven forward by firing pin. Pressure builds, front of case, which is thinner, expands and grabs side walls of chamber. At max pressure the case, which is till has the wiggle room expands to fill the entire chamber. Since the thinner brass near the neck is held tight by pressure to the chamber walls all that can happen os for pressure to drive the head end of the case back against the forward end of the bolt. Thos makes the case a bit longer but the stretching occurs just in front of the web of the case where the much thicker web meets the tapering sides of the case.

A rifle with a bit of flex in the bolt, like a rear locking lever action, may make this worse as the entire bolt may move a thou or so rearward which lets the case growth occur a bit faster.

This isn't the best or most scientific explanation but it describes things the way I see them.

I don't shoot max pressure loads much and don't push my case shoulders back when sizing so my shance of a separation is lower in my Marlin 30-30. There are ways to reduce the rate of growth but I don't think anything can totally stop it.

[dmize]

I would think that it would be the entire case being squeesed foreward. BUT with reverences to case head seperation and checking cases for a groove with a paper clip just up from the base. I dont know.

[Chicken Thief]

That's why brass stretching goes away when you only use a Lee Collet sizer.
When fired the brass fills the chamber and after that you only tighten the neck to get tension.

But the second post is correct.

[303Guy]

On firing the case expands against the chamber walls gripping it tightly. This occurs while the case is held forward by the firing pin. As pressure rises, the case head is forced back firmly against the bolt face. If the case grip is too great, the only place for the case to elongate is the web area and so thinning occurs there. That's because the elastic limit of the brass has been exceeded. If however, the case grip on the chamber wall is less intense, the case wall can 'creep' on the chamber wall allowing the elongation to be spread over the entire body length of the case, keeping the brass within the elastic limit and permanent elongation does not occur.

If there is clearance at the shoulder then that clearance will be forced out under pressure after the case has gripped the chamber wall, forcing brass to permanently elongate forward without thinning the web area.

[44magLeo]

I have an older Hornady book, Vol II. It has a very good explanation of what happens when you pull the trigger.
In btroj's explanation is good.
When the firing pin hits the primer it does move the case forward until the shoulder hits the front of the chamber. If the case has proper head space this movement is very small to none.
In this situation the case upon firing expands the front of the case and pushes the head back against the bolt. With minimal to no stretching.
If the head space is larger than it should be then the case stretches more and this stretch is mostly at just in front of the case head.
The heavier the load or the more the head space the more the stretch.
The head space of the cartridge in the chamber is controlled by case resizing. If you full length re size a too much you can push the shoulder back. This can lead to excess head space.
To set your dies to minimum sizing I like to smoke the neck of a lubed case. Now with the sizing die backed out a few turns, size the case, When you pull the case from the die you can see the smoke pushed down toward the shoulder. Now turn the die in , repeat till the smoke just gets pushed down till it almost touches the shoulder of the case.Now clean the case and try it in the rifle. It should close the bolt with no resistance, if any turn the die in at 1/8 turn steps until the bolt just closes with no resistance.
This keeps the shoulder of the case as far forward as possible and still function in your rifle. This is the minimum head space setting for those dies to fit your rifle. Different dies or different rifle and the settings may change.
When full length resizing as you squeeze the case back to size the brass has to go somewhere. It flows toward the thinner part of the case, that's the neck. It flows to make the case longer and the neck walls thicker.
If you are trimming very often, say every three reloads then your loads may be a bit hot.
You might want to measure a fired case from a factory or once fired case at the neck, write this down some where. After a few reloads measure the reloaded case at the neck. If it is very close to the before size then the neck is getting thicker.
If it gets so thick the neck of the loaded cartridge is the same size as the chamber, when the cartridge is fired the case neck has no place to expand and release the bullet.
This creates excess pressure. if you keep the dies set right and don't try to use max+ loads then you should get much longer case life with minimal trimming.
Leo

[cbrick]

This isn't the best or most scientific explanation but it describes things the way I see them.

No, actually that is a very accurate explanation. Good post.

Rick

[runfiverun]

b troj has it that would also explain why you have primers standing tall when you have reduced loads.

[EDG]

A sized case has some clearance that permits it to fit in a chamber. When fired the case expands like a balloon inflating until it is constrained by the chamber and the breech face. That expansion stretches the brass diametrally. It also stretches it longitudinally if you do not set up your FL die correctly. That stretching of the brass thins it slightly because it occupies a larger form when expanded. When FL sized the brass cannot be compressed back to its original thickness so it lengthens and the excess goes up the neck of the die.

[762 shooter]

Thank you gentlemen.

762

[Char-Gar]

The thinning of the brass above the case head comes from the brass being stretched as the shoulder of the case moves forward to contact the chamber.

The brass which accumulates at the case mouth and need trimming flow there under heat and pressure from the neck and shoulder of the case.

Please read my disclaimer below.

[leftiye]

There are two things in operation here. Case separations occur when the case grips the chamber walls under pressure and the headspace or flexion of the breech face pulls the case head away from the rest of the case. Brass is stretched (and compresses) under sizing and neck expanding operations. Making a brass tube smaller generally makes it grow in length also.

[405]

The posts that attribute most of the stretching to the thinner upper walls gripping the chamber, then the base section gets pushed backwards against the bolt face are correct.

There are instances where cases actually shorten, like when medium pressure fireforming a parent shouldered or bottleneck case in an improved chamber.

Also, while the firing pin can impose forward thrust to the case, it is the primer impulse that accounts for most of the forward movement. If it were the firing pin doing most of this work then how can a primer be left protruding fully rearward to the bolt face after firing a light load where the cartridge or gun have minimal headspace control?? One post did point out the protruding primer indicator.

Oh, and I think leftiye also brought up the "extrude" or flow part of the OP. Can't prove it- just going by various reloading/ballistic/lab sources- but brass can extrude or flow forward (upper body, shoulder and neck area) but that doesn't happen to any degree until really high pressures are exerted... like 50-60K+ PSI.

[btroj]

This also is why a good rule of thumb is to have the bullet into the lands for fireforming a case. Keeps the head tight against the bolt and make for front of the case blow forward rather tha the head of the case move back.

For most cast loads this is not really an issue. Many of us don't use lods that develop the pressures required for major brass flow near the head. This is also why we tend to see more soot on cases than shooting jacketed.

[DLCTEX]

Case neck stretching is aggravated by a die that undersizes the neck and then expands it back out dragging the neck longer, or dry/dirty necks dragging on the expander ball.

[Char-Gar]

This also is why a good rule of thumb is to have the bullet into the lands for fireforming a case. Keeps the head tight against the bolt and make for front of the case blow forward rather tha the head of the case move back.

For most cast loads this is not really an issue. Many of us don't use lods that develop the pressures required for major brass flow near the head. This is also why we tend to see more soot on cases than shooting jacketed.

The issue we cast bullet shooters can face with very light loads is having insufficiency pressure to have the brass grab the chamber. When this happens the firing pin blow can knock the chase shoulder into the chamber shoulder. Do this a few times and the case can have the shoulder back far enought to have headspace issues when the case is fired with higher pressures.

I dont use loads light enough to do this, but it is a a potential issue for some folks. Cases that shorten enough this way can in time give ignition issues, due to the weaker firing pin hit.

[leadman]

Another thing on some guns the area at the head of the case is not in the barrel, like on a Mauser military rifle. There is a slight gap between the end of the barrel and the bolt head.

Ever read the advertisement that Remington had years ago "three rings of steel" ?