6.5 Creedmoor brass keeps getting shorter??

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45C

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Hello
I started with 100 pieces of new Lapua 6.5 CM brass all measured 1.910.
After 4 firings they are now measuring 1.886 to 1.900.

I’ve never trimmed, only chamfered and deburred.

So, is this going to be an issue? Will they keep shrinking?

Is my chamber to long???

I’m using a rem 783 ,short barrel.
40.2 g of Varget
123 grain SST
Shooting 100 yard groups around.6 -.7.
 
What kind of dies are you using? If you're neck sizing only, this is normal, since the case is gradually "growing" to meet the chamber walls, therefore it gets shorter. It'll only get longer when it has no where else to go, or you full length size it.
I full length size every firing, I usually get 3-4 firings before I need to trim.
 
45C said:
I FL every firing.
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That's odd. Ok, so I don't know why that is..lol

Edit : I will say that the shoulder on your brass is moving ahead with each firing, so that will cause the brass to get shorter. At some point it'll start touching the chamber and become hard to close the bolt. Then, when you adjust the die down to start pushing the shoulder back, the brass will get longer.

It's possible your headspace is on the generous side, too. Are the primers in your fired rounds protruding from the case at all, or are they flush with the head?
 
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Primers are all flush after firing.
This the first bolt gun that I’ve experienced brass getting shorter, usually it grows and I have to trim.
 
If you have the tools to do it, measure from the shoulder datum to the head on both fired rounds and resized cases.

In the case of a 6.5 Creedmoor, the datum is a .375" dia circle located on the shoulder. The Hornady head space gauge is one way to measure for this dimension. I use a home made gauge with a .375" hole bored in it.

My new Lapua 6.5 CM cases, using my gauge, measure between 1.556" and 1.558" from head to shoulder datum. In my rifle, a Savage 10T, anything longer than 1.561" is a snug fit, I.E. the bolt is tight closing..

It's important to remember that each different gauge will give a slightly different dimension depending on the tolerance it's made to. Not an actual go, no-go gauge, but a gauge designed to measure the length of a case from the shoulder datum to the head.

It sounds like your headspace may be a bit long. If that's the case, it'll take several firings with moderate loads to stretch the shoulder forward to the point it touches the front of the chamber.
From that point on, the case will get physically longer as it continues to stretch.

I can control where the shoulder ends up by moving the die slightly up or down, but if the shoulder of the die isn't touching the shoulder of the brass, it can't push it back.
I use the 0.002" - 0.003" bump rule of thumb. From the longest head to shoulder dimension that will comfortably chamber in the rifle, I want to be 0.002"-0.003" shorter.
 
My guess is the factory chamber is a bit on the chubby side so the case is 'shrinking' as it fills out. At some point that stops and brass flow moves the length longer.

If the fired cases are a bit short, really doesn't matter as the slight change in length does no harm.. and maybe you will not have to trim the brass.

if the rifle operates properly and keeps shooting this well, just enjoy it. Anneal the necks every 3 to 4 firings and enjoy.

Jerry
 
Mystic Precision said:
My guess is the factory chamber is a bit on the chubby side so the case is 'shrinking' as it fills out. At some point that stops and brass flow moves the length longer.

If the fired cases are a bit short, really doesn't matter as the slight change in length does no harm.. and maybe you will not have to trim the brass.

if the rifle operates properly and keeps shooting this well, just enjoy it. Anneal the necks every 3 to 4 firings and enjoy.

Jerry
Click to expand...
That’s what I’m thinking as well.
They’ll have to grow quite a bit to need to be trimmed.. lol
Can they get to point where they’re to short??
 
I was going to say the opposite of Mystic Precision, meaning you have a skinny chamber at minimum diameter and a fat die near max diameter.

My thoughts were the case is not being reduced in diameter enough to make it grow in length. I say this because a larger diameter military chamber and a skinny die will really make the case grow in length.

What brand resizing die are you using?

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bigedp51 said:
I was going to say the opposite of Mystic Precision, meaning you have a skinny chamber at minimum diameter and a fat die near max diameter.

My thoughts were the case is not being reduced in diameter enough to make it grow in length. I say this because a larger diameter military chamber and a skinny die will really make the case grow in length.

What brand resizing die are you using?

wm05ArY.gif
Click to expand...

Redding
 
Chambers, dies and cases vary in size, the SAAMI cartridge and chamber drawing for the 6.5 Creedmoor allows .002 variation in chamber diameter, and .008 variation in case diameter.

So if your cases are not growing in length after full length resizing it means the case is not being "squeezed" enough to make the case grow. Example I have a Lee .223 die that reduces the case diameter more than my RCBS small base die does. I stopped using the Lee die because of this and trimming so often.

So borrow or buy another 6.5 Creedmoor die and size some of your fired cases. I would recommend a Forster full length bench rest die with their high mounted floating expander.

UcbgT7g.jpg


ZNXc0Hf.jpg
 
Question, what is the fired case body diameter at three points along the body and what is the body diameter after sizing at the same points.

Meaning how much is the case diameter being reduced after sizing.
 
bigedp51 said:
So if your cases are not growing in length after full length resizing it means the case is not being "squeezed" enough to make the case grow.

So borrow or buy another 6.5 Creedmoor die and size some of your fired cases.

UcbgT7g.jpg


ZNXc0Hf.jpg
Click to expand...

I don't see much point in doing this, other than as an experiment.
It's possible your headspace is at (or past) the outer limit of specs, and your brass hasn't stretched enough yet to fill the chamber. Until it does, it'll continue to get shorter each time you fire it as the shoulder moves ahead and the body of the case gets longer.

If your die is too large in diameter or too long to push the shoulder back, eventually you'll have problems chambering resized brass. I doubt that's the problem.

If you still have concerns or questions as to what's going on (I know I would, not knowing would drive me nuts! ) I'd suggest checking your head space with go and no-go gauges. A set should be less than $100. I don't have a set for my 6.5, yet. I do have .308 gauges, since I had my Tikka re-barreled. Just something I wanted to check for myself.

I don't think anyone has asked this yet, so I will. Are you measuring your case length before or after resizing?
 
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