Saco extractor

[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.

[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..

[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]

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..

[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.

[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.

[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...

[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]

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.

Lets use a 30-06 as our example.
Look at the surface area of the inside of the case.
This includes the case neck, shoulder, body, case head and the base of the bullet.
The internal surface area of the case body GREATLY exceeds the surface area of the neck as well as that of the case head or the base of the bullet.
The pressure from the burning powder acts on ALL of these surfaces.
Since the case body has the most area and the pressure acting on the area is equal in all areas, the case body will
be most susceptible to elongation and deformation.

The thinner areas of the case will have the most elongation and the thickest areas will have the least.
That's just the basics of how metal behaves under pressure.

The case NECK can not actually expand until the bullet moves forward and lets the pressure act upon the inside of the neck.
The pressure acts outward on the inside of the case though long before the bullet moves forward.

I can't help you if you choose to believe that the case BODY is not expanding and that just the neck is expanding and holding the case into the chamber as that sort of belief defies rational logic.

[MongooseV8]

Well obviously the case body expands, I never said it doesnt. And isnt the thinnest part the neck and shoulder? Wouldnt those expand the most as you point out? And the bullet can move out of the way pretty fast exposing the neck to the building pressure.

Like I said this isnt something I made up on my own, but at the end of the day this whole topic is speculation anyway. We cant exactly stand inside a chamber and watch in slow motion.

[diver160651]

Lets use a 30-06 as our example.
Look at the surface area of the inside of the case.
This includes the case neck, shoulder, body, case head and the base of the bullet.
The internal surface area of the case body GREATLY exceeds the surface area of the neck as well as that of the case head or the base of the bullet.
The pressure from the burning powder acts on ALL of these surfaces.
Since the case body has the most area and the pressure acting on the area is equal in all areas, the case body will
be most susceptible to elongation and deformation.

The thinner areas of the case will have the most elongation and the thickest areas will have the least.
That's just the basics of how metal behaves under pressure.

The case NECK can not actually expand until the bullet moves forward and lets the pressure act upon the inside of the neck.
The pressure acts outward on the inside of the case though long before the bullet moves forward.

I can't help you if you choose to believe that the case BODY is not expanding and that just the neck is expanding and holding the case into the chamber as that sort of belief defies rational logic.

Thanks everyone. I think I understood most the process of expansion -- hardening etc.

Sorry I took me sooo long to get the explanations.

Except honestly, was thinking of this backwards.. Thinking the bolt face was ground zero..

FWIW, I seldom need to trim brass ~

Talking about (1)-brass sealing the chamber.. or being a "gasket" and (2)-Lynn's mention the wrong round in a chamber, is exactly what (1)-didn't and (2)-did happen.. Funny how there is only so much elasticity in the brass when you put a long thinner round into a fat chamber... I was calling wind on the gun and without lifting my head felt for my round and single loaded the wrong round.

[kendog4570]

You guys should get a room...
At the risk of coming off like The Guffster, one can also "stretch" bottleneck cases with an improperly adjusted FL sizer.

[ar15barrels]

And isnt the thinnest part the neck and shoulder? Wouldnt those expand the most as you point out?

I should not have said the most.
The chamber obviously limits the expansion amount as the case stops expanding when it comes to rest against the chamber.
Think of the case as a pressure activated gasket that seals up against the chamber when the pressure acts on it.

The thinnest section expands the easiest.
The amount of surface area will determine how much force is present in a given area.

Pounds PER Square Inch.

Using our 30-06 example, the inside of the case body has around 2 square inches of surface area.
At 55,000 PSI there would be about 110,000 pounds of force locking the case against the chamber.
The inside of the shoulder area would receive about 6900 pounds of force, with more than 2/3 of that force being outwards and less than 1/3 being forwards.
The base of a 30 caliber bullet would receive about 4100 pounds of force.
The inside of the case neck once the bullet has left would receive about 20,000 pounds of force.
The inside of the case head would receive about 6800 pounds of force.

The inside of the primer cup ends up with about 5900 pounds of force.
This backs the primer out of the case head against the breech face until such time that the case body yields and lets the case head slide back against the breech face.

Low pressure loads will often show a protruding primer after being fired.
This is because the pressure was not high enough to yield the case body and the case head did NOT slide back.
The primer cup will be sticking out by the amount of headspace present in the chamber with that particular case.

Because of the much larger surface area, the case body will elongate and yield long before the bullet moves forward out of the case neck, allowing all the pressure to reach the case neck.
Once the bullet moves out of the neck, the neck is also contributing to holding the case forward in the chamber though.
It's just a much smaller piece of the overall pie.

[MongooseV8]

Yeah Im with you on the psi vs surface area.

[LynnJr]

Load your ammo like Benchrest shooters and you will never have a problem.

And I think we all got insulted by KenDog. Calling us Guffsters Now that is insulting.