Case length variation in new brass and accuracy

[Les in Wyoming]

I am sorting out my brass and started by measuring the over all length of the case. My once fired cases vary as much as .014. So then I went to a new bag of cases and found the difference to be the same. Most are all under the spec length (which I take to be the recommended limit length). I don't remember if I even messed with case length, but rather focused on cartridge overall length once the bullet was set. This is for a 300 WM and I like sub 1/2 MOA. The spec in the book is 2.620. So, these are my questions: 1) How critical is case length? 2) What tolerance of variation do you allow? 3) how much below the 2.620 should I consider too much?

My hunch right now it to trim them all to 2.610 and throw away all others. But if 2.605 is OK, it would save me a lot of brass. It all depends on how critical this is. I greatly appreciate input. Thanks. Les

 

[Dmagna]

Don't throw anything away, load it and shoot it. See if it groups the same, if it does group then it doesn't matter. I have never had case length effect groups. But remember it will grow so it will be fine in the end.

 

[cape cove]

Get yourself one of these and find out your chamber length and trim .010 under. They work great. As posted above keep all the short ones, they will grow.

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[RegionRat]

1) How critical is case length? 2) What tolerance of variation do you allow? 3) how much below the 2.620 should I consider too much?

1) This is one of those "it depends" answers. Many accomplished BR shooters and Highpower shooters will debate the topic at length, with some saying it has proven critical and others claiming it makes little difference.
Jack Neary mentioned a story how important is has proven to him in those video taped lectures he gave. I don't shoot BR, but I will say that it is just as easy to keep this topic under control as compared to the work it would take to investigate.
At least two completely different systems exist, ones that index the cut from the base, and ones that index the cut from the shoulder. Both work well for me and I can keep case batch length variation to less than 0.003" with very little effort.
ETA: I forgot the kind that index off a rod that runs on the bottom of the case web, which I would never use.

2) If the cut is from the base it usually comes out less than 0.0015", and from the shoulder it comes out less than 0.003", that is after chamfer and tumble.

3) You can pick your system. You either measure your actual chamber, or go from specifications. I keep all of them to within 0.010" of the recommended max, but a batch will be within less than 0.003". That happens because I set up the trimmer and run them all over the trimmer every cycle, and also chamfer after every cycle. On a rare occasion I may get away without trim, but I still chamfer ID and tumble to remove lube. YMMV.

I will add that some specifications allow more than 0.010" for case length. I will pose a question for thought. What do you suppose happens if a carbon ring builds up near the end of the case mouth in the chamber, and then a longer than usual case is used? Not all hunting chambers have the same neck clearance designs, but I think along with trimming, it is best to clean your chamber area well enough to prevent carbon or hard residue from building up in the neck area. YMMV.

 

[QuietTexan]

First, only use matched brass from the same lot. Don't worry about new out of the box measurements other than not being overtly and plainly out of spec. Usually I drop cases in an LE Wilson case gauge out of the box and if they fit they get fired. About the only cases that I cull when new are ones that don't drop in at first, and then don't drop in after a trip through a sizing die.

Second, fire them at least twice before you move the shoulders or trim them. If you inspect new brass and the shoulder measurement varies by .002-.003 then you would expect the overall case lengths to change as the shoulders blow out when fired. So by trimming before the case is fully formed you're actually introducing a variance you'll have to cut out again later.

Third, measure your actual chamber (as mentioned before), trim them to work with that if they ever need it. If they don't, then I don't. Big caveat - I shoot a lot of improved chambers/35-40* shoulders that stay short on their own, 30-06 is about all that I really need to trim for fitment issues because the shallow shoulders do grow a lot no matter what. If you're shooting shallow shoulder angles then you'll need to trim more frequently.

One important thing: when/if cases need to be trimmed, check how square the necks are. Especially if you previously trimmed to a square end and then have to trim again. Uneven neck growth can be an indicator of thickness variation, and cases that grow unevenly can have more issues than just in the necks like capacity variances.

I wouldn't bother sorting by case length, there are several other data points that are significantly more important and much harder to change.

 

[Les in Wyoming]

Many thanks for the wealth of information here. I have learned much by this forum. I used a lee tool that hits the bottom of the case at the primer hole and it is set to 2.610. It is easy to over hog material on this, however. Some ended up 2.605. They are in the "shooters" pile. Thanks

 

[L.Sherm]

I can assure you .020 shorter brass from max trim length will shoot just fine, you hear the arguement about carbon ring but ask yourself do you really think cleaning a .010 ring vrs a .020 matters or can you tell the difference.
I've built 2- 6.5x06AI that both shoot Lapua 06 brass that are .020 shorter than max length and both will shoot 2- 2.5" groups at 720 when I do my part.
Trim them all to 2.605 and be done, dont let them get to long. Quality of brass matters more than if there .010 or .020 shorter than max length.

 

[coop2564]

Keep and shoot the brass. My experience shows little to no difference in groups or velocity in the discrepancy you are talking about here.

 

[RidgeTop]

What mfg. of the brass?

I spot check new brass and it is amazingly consistent.

Why don't you fire form it all, then think about trimming or discarding any too short.

I use a pistol powder and corn meal to fireform. About a 1/2 teaspoon of unique and an ounce of corn meal topped with a little cotton to hold it close to primer (keep barrel tipped up). I used this for .303's. You will need ear plugs.

 

[Lenny Foffa]

I used to trim a lot ! 10/1000 under factory spec as suggested by the manuals,then I bought a Sinclair chamber length gauge ! In both of my 270 win rifles the chambers. Measured way way longer then factory speck ! I was trimming for no good reason ! I am getting the Sinclair gauges. For every caliber I shoot !!

 

[MagnumManiac]

I ran several 300WM rifles, a few factory but mostly comp guns.
I never ran my brass at 2.610". All of my brass was run at .010" below MAX CHAMBER LENGTH AND KEPT THERE.
One of my barrels liked 2.645" but most ran 2.640".
Also, brass only stretches when you size it, not from firing. When you fireform a belted mag, generally the neck SHORTENS when you fire it.
I take little from the actual length on group size, but whatever it is, it must be SQUARE to produce consistent groups.

Cheers.

 

[DUSTY NOGGIN]

Many thanks for the wealth of information here. I have learned much by this forum. I used a lee tool that hits the bottom of the case at the primer hole and it is set to 2.610. It is easy to over hog material on this, however. Some ended up 2.605. They are in the "shooters" pile. Thanks

try these , cheap and exact same every time , pilot bottom out through the fashhole like the leee , plus the drill is on the correct side of the operation , get the rifle kit and and additional pilot for the 300wm

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[DUSTY NOGGIN]

Also, brass only stretches when you size it, not from firing. When you fireform a belted mag, generally the neck SHORTENS when you fire it.

Cheers.

speaking of Fguffey , summed up all in one sentence..... IYKYK

 

[Bob Wright]

I noticed as much as .005 growth after sizing on 7RM Remington brass on my last brass prep.