Trimming case length, how much does it really matter?

[Bigeclipse]

Ok reloading experts, I know Case length can matter for safety purposes. So for instance my reloading manual states for 3006 the case itself should not be longer than 3.494 (I think, I do not have my book on hand though. That is from memory). When you fire your rounds, the cases can stretch past this point and you should now trim them back a bit. First question is, if your manual states like my example 3.494, how much will you trim it to? 3.494 exact? 3.490, 3.492, 3.485....etc? My last question is, if you have cases trimmed differently say a couple are right around 3.493 and a few others are slightly shorter at say 3.485, what kind of impact will it have on accuracy? Small/large? Im sure for competition purposes or extreme long range hunting this is probably super important, but to try and maintain a load around 1 MOA out to 400 yards how critical is this? Again this is not for the total cartridge OAL but simply the brass case itself.

thank you.

 

[CB11WYO]

Well for question #1... I trim to .010" under the "max case length." So if your 30/06 shows a max of 3.494" I would would trim to 3.484" and so on...

As to the effect on accuracy... I've never tested that specifically. I do know that after 2-3 firings of typical cartridges (22-250, 7mmRM) I only see a measured difference of .005" or less between cases when taking a sample of 20% of my cases. In other words after firing 2-3 times my brass all measure within .003-.005" of each other. That spread is even less with brass from, say my 20 BR. Would be interesting to know of the accuracy implications though...

-Clint

 

[Mikecr]

Well I don't do anything by a book. I measure the chamber end length with a gage and trim from there.
I set -5thou from end where I can.

For your expectations, being way short of chamber end probably won't matter. Being long will matter and it can be dangerous.
The price for being way short(like +10thou) is increased velocity spreads(ES). With more clearances, gas can better work it's way around necks/shoulder prior to case sealing. It's an angle thing.
This adds a variance to case sealing & so to your peak pressure curves.
It's not a lot, but one of the matters in reducing ES.

A 30-06 has to be sized & trimmed continually. You're left to manage this, so don't even bother to try staying long without getting in trouble. Just pick a value that's conservative to your management of it. Maybe 10-20thou.

 

[Bigeclipse]

Well I don't do anything by a book. I measure the chamber end length with a gage and trim from there.
I set -5thou from end where I can.

For your expectations, being way short of chamber end probably won't matter. Being long will matter and it can be dangerous.
The price for being way short(like +10thou) is increased velocity spreads(ES). With more clearances, gas can better work it's way around necks/shoulder prior to case sealing. It's an angle thing.
This adds a variance to case sealing & so to your peak pressure curves.
It's not a lot, but one of the matters in reducing ES.

A 30-06 has to be sized & trimmed continually. You're left to manage this, so don't even bother to try staying long without getting in trouble. Just pick a value that's conservative to your management of it. Maybe 10-20thou.

thanks but I think i explained my question wrong. I dont mean should I leave them long I meant more lets say I have 5 cases I will use to shoot a group. Lets say 3 cases were trimmed to 3.490 and the other two trimmed to 3.483 (both under dangerous length)...would group/accuracy suffer from the .007 difference in trim between the cases? My guess is yes but to what extreme? I know it probably "varies" from rifle to rifle but if I am trying to keep my rifle around 1moa or less, do I really need to worry about sizing all my cases exactly the same? thanks!

 

[Mikecr]

Yes you need to size them all the same, your trim length in itself(~10thou diff) will not be significant to 1moa.
Hell, anything not completely broken will shoot 1moa to 400yds.

 

[MagnumManiac]

Unless you have a highly tuned rifle with tight tolerances, you will see no difference in group size, and even then it depends on more than a few factors.
When fireforming for improved cases, the neck always shrinks in length as the case shoulder pulls brass out of the neck as it expands. A few loads later, the neck has grown and a trim may be in order, not once have I seen a difference in group size from this in a hunting rifle, but in my 22-250AI there is a very slight but noticeable difference, but I feel this is due to neck tension differences rather than any other reason. The neck expands from the shoulder forward upon firing, so the length should not influence the accuracy directly other than having less neck tension.

Mikecr,
I don't see how a shorter neck creates more clearance for gas to find a way past the neck, with the correct pressure range the amount of gas bypassing the neck should actually be less, not more. Only on a few occasions have I seen gas marks that have gone more than 1/10" down a neck, and this has always been only on one side, this fact I have no answer for, but is my observation in quite a few cartridges. One odd observation I have is that true big bores are crazy accurate with no tuning, truing or blueprinting necessary, again, I have no answer for this fact either.

Cheers.
gun)

 

[Mikecr]

There are further qualifiers to gas sealing and it's only one factor. It does depend on cartridge & load.
And it does 'seem' like a neck mouth further from the throat would see less gas before sealing.
But often reality is just opposite.

If you take something like a 243Win, typical neck clearances and seated off the lands, load it LO-pressure, and vary trim lengths, you'll see sooting follow. It can go from little to half way down shoulders or more as you trim more & more away.
Sometimes the gas blow-by can pocket and cause a case dimple.

As with all things, there are prices for them.