what difference does brass length make?


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coosbaycreep
March 5, 2010, 01:45 PM
I didn't see an answer to this in any of my reloading manuals.

They all list a max length and a trim length, but as long as all the cases are shorter than the max and fit the chamber, how important is it to trim all of the cases to the same length?

Do any of you just seat the bullets so that the COAL is the same on all of the rounds, even if there's slight variances in the actual brass length?

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pingpingping
March 5, 2010, 01:47 PM
consistency is the key to accuracy

coosbaycreep
March 5, 2010, 01:56 PM
That makes sense.

Accuracy aside though, as long as the case is somewhere between the trim length and the max length, there's no problem with overpressure or anything though, right?

I was mainly wondering on the basis of safety.

Walkalong
March 5, 2010, 01:58 PM
Unless you change the seater setting in the middle of things, the bullets will all be seated the same in comparison to the lands, regardless of case length. There will just be more or less of the bullet in the neck.

As posted, consistency is the key to accuracy, so it makes sense to trim rifle cases.

I trim revolver cases and all rifle cases.

That said, as long as the cases are under max, they will be safe, and shoot quite well.

coosbaycreep
March 5, 2010, 02:07 PM
That's what I wanted to find out. thanks

rcmodel
March 5, 2010, 02:18 PM
Carried to extremes, a too short case exposes part of the chamber to gas erosion just like the rifling leade & rifling.

In a center-fire rifle, in theory, continued use of too short cases would result in chamber erosion, and eventually, a reverse bottleneck case neck area of the chamber that would prevent extraction of the fired case.

Has it ever happened?
Probably not, but in theory, it could.

The other thing is, if you are going to crimp using a standard seating die, all the cases have to be the same length.

rc

Clark
March 6, 2010, 12:20 PM
If the case is too long, when the bolt pushes the case in the chamber, the neck will pinch the bullet.

If you liked a 5k psi increase from jamming the bullet into the lands, you will love a 20 kpsi increase from pinching the bullet.

The good news is the max case length in load books is almost certainly shorter than the chamber case length in you rifle.

The bad news is that if you pushed the shoulder back, the difference between the case head space and the chamber headspace is how far forward the firing pin will push the case. That firing pin can actually push the case hard enough into the end of the chamber to get some bullet pinch.


This phenomena of large bullets working in a small bore without
pressure spikes is documented in P.O. Ackley 1966 "Handbook for Shooters
and Reloaders Vol 2" chapter 7
"additional pressure tests":
..30 cal barrel pressure barrel was fitted to the test gun, but the
neck and throat was enlarged to accept the 8mm bullet, with the bore
remaining the standard 30 caliber. A Remington factory 30-06 cartridge
with the 150 gr bullet had been tested and previously gave 57,300 psi,
for a velocity of 3030 fps. The the bullets were pulled from two more
Remington 150 grain cartridges and were replaced with 8mm 150 grain
bullets. To everyone's surprise, although the velocity was rather
erratic, these loads averaged 2901
fps, with a pressure of 40,700 psi.

What does it all mean?
For most people it is counter intuitive that large bullets swage down without noticeable pressure spike, while bullets pinched by case necks make huge pressure spikes.
And brass that is too long can pinch the bullet.

Dave Bone
March 6, 2010, 12:44 PM
What type of ammunition are we talking about here? You didn't mention what type of case. Rimless straight wall is the only type where case length determines head space.

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