Saco extractor

[MongooseV8]

Randall explained it well although I understood one aspect a bit differently.

After ignition the neck expands and locks into the barrel/chamber, then the case head slams rearward into the boltface, then the walls expand to the chamber. Read that in a loading manual a while back.

But either way the unsupported weakest part of the case stretches with each firing near the case head. You cant really stop it completely but there are techniques already mentioned to help mitigate the problem.

I have some Ackley Improved cases with well over 20 loads through them due to the straight wall chamber. The standard taper case and chamber is great for reliable feeding but makes it difficult for the case to grip the chamber upon firing and stretches out the brass.

[ar15barrels]

Thanks guys- I have 21st century expander mandrels. That false shoulder is starting to make sense for fire-forming the shoulder, but doesn't the brass still want to also grow from the case head?

Brass does NOT "GROW" from the case head.
What happens in a chamber is something like this:

Loaded cartridge is placed in the chamber by the bolt.
The bolt is locked closed.
The spring loaded ejector pushes the case forward against the headspace feature. (rimless cases usually use the shoulder as the headspace feature).
In this condition, all of the HEADSPACE is between the case head and the bolt face.
Now the firing pin strikes the primer.
The burning powder creates pressure inside the case.
The portion of the case FORWARD of the thick part of the case head will expand and STICK to the chamber walls from the operating pressure of the cartridge.
IF there is enough headspace and enough pressure to stretch the case, the case will stretch rearward until it comes to rest on the bolt face.
What you are calling GROWTH is actually the case stretching to fill the headspace.

Remember that the front portion of the case is STUCK to the chamber at this point.
The stretching will occur right where your cases are splitting.

This is why you want to keep the headspace as short as possible by sizing the cases to JUST let the bolt close freely.
Do not push the shoulders back any more than necessary.
By reducing headspace, you will reduce the amount that the cases stretch.


If you learn about how metal behaves, you will learn that most metals (as used for cartridge cases) will stretch some BEFORE they YIELD.

The amount that a metal temporarily stretches or bends, but then returns when the pressure/strain is removed is called elongation.

When you pass the elongation threshold, you get permanent deformation.
Permanent deformation is the change in dimension that does NOT go away when the pressure/strain are removed.

By keeping headspace near zero, you can actually reach a point where you have NO plastic deformation of the case during the firing cycle.
This is how benchrest guys get away with simply depriming and seating a new bullet without actually sizing the case.

Close fitting of brass to chamber like that is usually not a good idea in a field gun though so you have to compromise your brass fit to ensure the gun will run when the wind blow's some of Parma Idaho's finest into your action...

[ar15barrels]

The extractor doesn't hold the case against the bolt face?

No.
The extractor usually has around 0.005" to 0.010" of clearance to allow the case to PIVOT against the ejector when the case is ejected from the action.
Case rim thickness varies across different brands and lots so the extractor has to have some clearance over the THICKEST case rim that it will ever extract/eject.

Headspace runs between 0.000 and maybe as much as 0.004" on a gun that is in-spec using ammo at the long end of the SAAMI specs where it should be.
You can see that there is MORE extractor clearance than there is headspace so the extractor should never be touching the case rim when the bolt is locked in battery.

[LynnJr]

Diver
The false shoulder is what keeps the casehead firmly against the boltface. That is why we want the tight fit as well.
When combustion gases build pressure and everything expands to fill the chamber the neck and the cases overall length shrink.

If you bump the shoulders too much the casehead is no longer forced against the boltface.
When the firing pin in the bolt hits the primer it forces the case forward until the chamber or the extractor stops it.
The combustion gases push the case in all directions and pin it against the chamber walls.
At peak pressure the unsupported casehead gets forced rearward while the rest of the case is still pinned against the chamber and this is why you get thinning right in front of the web area.
The brass is just a gasket for the chamber so it can seal itself. Brass as you know is pretty soft stuff.
A tell tale sign you are doing something wrong is when you constantly need to trim your brass.

[diver160651]

If the headspace is correct, the brass does not grow from the casehead. At the moment of combustion the pressure will push the casehead against the boltface (and should go nowhere since it's already against the boltface) and the brass will expand against the chamber, the only unsupported area is the shoulder which will expand forward.

If there is excessive headspace, at the moment of combustion, the pressure makes the case expand against the chamber and the casehead is unsupported which is then pushed back against the boltface, stretching the case, that is where you get that telltale sign of casehead separation about 1/4" from the head.

I assumed the bolt face to the edge of the shoulder was were stuff head spaced and thus grew always from the case head to the shoulder. But all these years I had it backwards apparently.

Thank you..

[buffybuster]

Thanks guys- I have 21st century expander mandrels. That false shoulder is starting to make sense for fire-forming the shoulder, but doesn't the brass still want to also grow from the case head?

Sorry for being so dense.... I think I have the wrong image/idea or whatever in my head and can't visualize how that helps a thing case head web thinning thing.

I appreciate the time you guys are taking on this..

If the headspace is correct, the brass does not grow from the casehead. At the moment of combustion the pressure will push the casehead against the boltface (and should go nowhere since it's already against the boltface) and the brass will expand against the chamber, the only unsupported area is the shoulder which will expand forward.

If there is excessive headspace, at the moment of combustion, the pressure makes the case expand against the chamber and the casehead is unsupported which is then pushed back against the boltface, stretching the case, that is where you get that telltale sign of casehead separation about 1/4" from the head.

[diver160651]

SigStroker
The extractor has give to it and you end up with 0.004 - 0.005 worth of slop.

Diver
As Buffy posted you run a mandrel or a K-M Expandiron and yes that is spelled correctly into the neck and enlarge it.
You then neck size it down until you get a very tight fit and fire away.

Thanks guys- I have 21st century expander mandrels. That false shoulder is starting to make sense for fire-forming the shoulder, but doesn't the brass still want to also grow from the case head?

Sorry for being so dense.... I think I have the wrong image/idea or whatever in my head and can't visualize how that helps a thing case head web thinning thing.

I appreciate the time you guys are taking on this..

[LynnJr]

SigStroker
The extractor has give to it and you end up with 0.004 - 0.005 worth of slop.

Diver
As Buffy posted you run a mandrel or a K-M Expandiron and yes that is spelled correctly into the neck and enlarge it.
You then neck size it down until you get a very tight fit and fire away. The area right at the neck/shoulder junction has a bulge and that bulge locks the case firmly.
90% of accuracy is the powder charge and the seating depth. Keep those two things consistent and you should be good to go.

[buffybuster]

The extractor doesn't hold the case against the bolt face?

Not tight enough to set correct headspace, especially if you have a plunger ejector pushing the case away.

[sigstroker]

The extractor doesn't hold the case against the bolt face?

[buffybuster]

I doubt I've loaded any rifle case more than 3 times, or pistol case more than 6 times. It's hard for me to imagine someone shooting more bolt gun rounds than I do machine gun rounds.

I know some BR guys running the same PPC cases for over 20 reloads.... and I expect most everyone running precision tactical to get 10 reloads before retiring if they don't loose them first. I don't count how many times I've reloaded pistol cases.

How do you great a false shoulder...

Typically you would neck up the brass to next larger size (6.5mm to 7mm, etc) then neck is back down. That will create a donut in the bottom of the neck which will be your false shoulder. The slightly fiddly thing is when you next down you want to just push the neck just far enough the case chambers with resistance. That will establish your headspace for the case. When it's fired the shoulder will blow out (fireform) to the chamber. I would then anneal the necks because they could be work hardened from all the sizing.

[sigstroker]

I doubt I've loaded any rifle case more than 3 times, or pistol case more than 6 times. It's hard for me to imagine someone shooting more bolt gun rounds than I do machine gun rounds.

[diver160651]

Saco extractor

Diver,

If bumping from 0.001" to 0.003" resulted in casehead separation, then that brass was on it's last legs. Jamming the bullets wouldn't have saved them.

If the brass was in good shape and you wanted to salvage what you could, then you could reseat the bullets longer to jam into the lands with good neck tension. Without enough neck tension, the case would move forward during the FP strike. It's not ideal but and option. The next better step would be to create a false shoulder but that's more steps. Lesson learned.

Ya, I knew it was getting old I track in MTM boxes and anneal after the 3rd firing.


Maybe I am becoming Superstitious but my over bumps happened on the 13th Reload.

The 14th firing was not the best.. funny how the bullet impacts a tiny bit low with a completely detached case head. :-(

It’s pretty depressing to load 20 boxes of 50 and realize you made a mistake on the last couple. But really it was time anyway.

Lynn, your several pay grades above me on the reloading stuff. Truth be told I am starting to slip into the good enough camp for most my shooting and as mentioned started using factory at least until my kid ships off to Wyoming for Collage.

How do you great a false shoulder... and while I still use a stony point to know were the lands are, for the kind of shooting we do the most, we have to be able to rack and unrack a live round if the clock times. Not sure a hard jam is in the cards. I have seen a couple over the years leave a round in the barrel and even jam a jewel with the powder.. not a good thing.

Ok, sorry for the dumb as questions.. at this point it is only academic. But if the shoulder is set back and the round is shorter than the chamber, while the bullet hard jammed, does that stop the shoulder from moving forward and flowing the brass?

Again, sorry if this seems anything but curious, I've been reloading some time and really seem not to be understand the concept.. Maybe I need a hammer to the head?

Sig that was really funny!

FWIW- I certainly have learned a lot in this thread

Jt


Sent from my iPhone using Tapatalk Pro

[sigstroker]

It can go fast on old brass

Wow, that one on the right got split clean in half.

[buffybuster]

Diver,

If bumping from 0.001" to 0.003" resulted in casehead separation, then that brass was on it's last legs. Jamming the bullets wouldn't have saved them.

If the brass was in good shape and you wanted to salvage what you could, then you could reseat the bullets longer to jam into the lands with good neck tension. Without enough neck tension, the case would move forward during the FP strike. It's not ideal but and option. The next better step would be to create a false shoulder but that's more steps. Lesson learned.