Case head separation

[jelenko]

If you can rechamber a fired case easily you should not be bumping from this dimension. Size with no bump until the case is tight or maybe 2-3 firings with no bump to get your bump from dimension. Don't know about case gauges (+/- 0.002" machining tolerance?) . I'm sure it's made to some std SAAMI dimension and not your actual chamber.

Right. Case gauges are made to be 'zero' at SAAMI min. That said, in my experience, all the case gauges I've had read +1 to +2 on a go gauge.

 

[bozo699]

All this nasty talk of neck sizing!… where’s Erik Cortina at when you need him?!….
Wayne

 

[SeabeeKen]

Are you saying the "bump" was moving the shoulder after every firing? In that case, then yes, that's the problem, You are moving a fireformed case up to .003" shorter than the chamber. Maybe you should think about backing off the sizing die to mirror the chamber dimension and only size the neck.

^^^^ those case have stretched .021”, thats do it.

 

[ebb]

^^^^^ Do we need him? I've neck sized and enjoyed it, and it produced high quality ammo.

 

[358WCF]

I've recently found a few of my lapua palma cases with signs of case head separation,
The brass has been fired 7 times, 2 times in 1 rifle ,5 in the current barrel and generally the load was 44gr of n150 with a 175gr running at 2730 from a 28" barrel
Most people have mentioned excessive headspace, I've measured numerous times with a whidden case gauge and the bump was between 2.5 and 3 thou almost always
Could there be any other causes ?

Most all new brass is on the short end of the spec so it fits everything. One lot of Alpha 7-08 from a year or so back was actually a tad too long, but this is the only brass I've ever had to run thru the FL sizer when new.

The initial firing can start the stretch causing the separation. I have some premium 280 brass that started coming apart after 5 or 6 firings, even being very careful when adjusting the FL die since the 1st firing, only bumping after the bolt got notably a bit stiff on closing. There's another untouched box from the same lot bought at the same time in the closet & sure enough they are .006 to .008" shorter than my chamber. Will neck it up to .30 cal & resize for a snug fit on the 1st firing. The lump at the neck/shoulder junction should hold it against the bolt face on the 1st firing & hopefully all will be good. Maybe this is what happened to you?

 

[bozo699]

^^^^^ Do we need him? I've neck sized and enjoyed it, and it produced high quality ammo.

Sure
Wayne

 

[Will Henry]

There are two things which can cause case head separations: excessive head clearance (from setting the shoulder back in sizing) and bolt deflection (generally in a rear locking action). In your case, it is due to head clearance. In a strong, front locking action, pressure is not a factor. Neither is reduction of case diameter in sizing. WH

 

[243winxb]

The brass has been fired 7 times, 2 times in 1 rifle ,5 in the current rifle.

1. Never use rhe same brass in 2 different rifles.

The initial firing can start the stretch causing the separation

Very true.


2. The first firing may stretch the brass .010" or more. The 2nd rifle may strerch brass again.

A crush fit, when closing the bolt, is rhe only way to know that the brass has fully expanded to the chamber.

A starting load may make the head to datum measurement shorter. More stretching as the powder charge is increased.

 

[Old Navy]

IMHO, I think repeated sizing with bumping that much can easily do that. . . especially if the die is sizing a lot down near the web.

If not that, then this ^^^^^

Because the chamber was cut too big.

 

[Sniperhandle]

I have seen so many of this type of thread since bump dies came out. Many people using them have no concept of how headspacing works. All kinds of problems due to not understanding it.

 

[bozo699]

I have seen so many of this type of thread since bump dies came out. Many people using them have no concept of how headspacing works. All kinds of problems due to not understanding it.

Since bump dies came out?…..you mean like over 80 years ago?….
Wayne

 

[cem]

Given that you have a borescope you can look inside the case at each firing and watch a bright ring develop inside and out. In my experience and reading case head separation is always and only too much headspace. The cartridge is pushed forward by the firing pin, gripping the walls on firing and the head pushing back into the available space. Other separations and splits can come from work or age hardening or ammonia exposure or other hard on brass issues.

 

[M-61]

Since bump dies came out?…..you mean like over 80 years ago?….
Wayne

If that’s when they came out…..then yes.
Hie second part of his post is right on the money. People not having the slightest idea of what is taking place when the trigger is pulled. Far better off buying factory ammunition. Too many having no idea and too many supplying erroneous or at least possibly dangerous information.

 

[bozo699]

If that’s when they came out…..then yes.
Hie second part of his post is right on the money. People not having the slightest idea of what is taking place when the trigger is pulled. Far better off buying factory ammunition. Too many having no idea and too many supplying erroneous or at least possibly dangerous information.

Possibly but I don’t see too much bad information being given on here and when it does happen it doesn’t take long to be corrected.
Wayne

 

[AlNyhus]

The chamber is the ultimate 'case gauge'. You should be setting the shoulder bump by using brass fired in your chamber.

 

[243winxb]

Take the maximum chamber head to datum measurement. Subtract the minumum cartridges head to datum.

In the 308 Win, this leaves a head clearance of .013" Possible damage to brass on firing.
If new factory brass/ammo has been drawn/formed correctly, along with annealing, there will be no problems or separations on the 1st firing.

Old military rifle, new ammo brass may separate on firing from excess head clearance. The reason most times in the bolt has been switched.

 

[M-61]

The chamber is the ultimate 'case gauge'. You should be setting the shoulder bump by using brass fired in your chamber.

Yes, but I will add “if needed”. An example in my situation is my 222. Maybe 20+ reloads. Never bumped the shoulder ( yet)
Is it easy on brass? Yup. Much like my 30br. Very easy on brass, and as far as the barrel goes, I wonder if you can shoot one out. Didn’t want to change the subject but I did.
My objection is Willy-nilly bumping of shoulders with out the slightest idea of what scenario you are setting up.