Trying to figure out where my case length deviations are coming from.

.308

New Lapua brass

Fire formed twice

Bump shoulder .002

Re size neck .006 in 3 steps

Trim in Giraud Trimmer

Now 10 cases processed EXACTLY the same show these OAL:

2.0080
2.0080
2.0080
2.0080
2.0085
2.0085
2.0105
2.0115
2.0120
2.0120

I know the Giraud Trimmer indexes of the shoulder so I measured all from base to shoulder and they are all the same.

I am using a Forster Bushing Bump die.

I am totally lost. Where are these deviations coming from?

Thanks

Your caliper likely resolves to +/-0.0005" so that possibly accounts for ~0.001" of the deviation. How well the tool is handled probably accounts for the rest.
Measurements this small are very hard to do accuratly.

.004 of an inch???

Sounds like a normal part of the trimming process to me. You can usually get that much variance just turning the case in the caliper.

Just because your caliper has 4 zero's on it, doesn't mean you have to use all of them.

You are applying too close of tolerance to your case length. Calipers are accurate to a point; micrometers are the base line for accurate measurements. Don’t get worked up over the deviation. You’ll never see a difference. Maintain a length in the “.000” range, you’ll do just fine for now (maybe forever) and you'll stop driving yourself nuts.

The variance would have to be from the trimmer indexing on the shoulder. There could be an ever so slight variation in brass thickness that would throw it off that little bit.

I guess the only way to know would be trim about 10 on another type of trimmer and compare.

If the trimmer indexes off the shoulder, are we sure that the press and the dies are setting the shoulders to a perfect length?

Dies have a habit of backing off (unscrewing themselves).

quote:

Originally posted by MickinColo:
If the trimmer indexes off the shoulder, are we sure that the press and the dies set the shoulders to a perfect length?

The press should be sizing each case the same. The comparator and trimmer should be contacting each case the same respectably.

Composition differences (thickness, hardness) in cases due to manufacturing or firings (charge weights) could be effecting spring back.

It was just something to check. If you feel good about it, I'm good with it too.

Anything is possible, that's why I said "should". thanks

A dull cutter will drive you bonkers.

2 cents

quote:

A dull cutter will drive you bonkers.

Soft brass will do that too....

Take all the cases that you believe measure the same. Load all with the same load and fire in one chamber. Now size all of them with the same die setting and then re-measure case length. There is a better than even chance you'll now find variance in case lengths...

So even if all cases could be trimmed to the same length, unless you are willing to trim after every sizing, you'll still have to suffer through cases with varying length.

- mike

I came across the fact that using the shoulder as the stop point would cause problems with OAL length when using the hand trimmers I bought somewhere...EVEN AFTER FL RESIZING in several different brand sizing dies...so I went back to base to neck end OAL measurements.

The only thing I could attribute it to was the fact that brass is slightly "springy" and each piece springs a different amount.

You are measuring in 0.0001" which has it's problems...WITH YOUR TECHNIQUE and THE MEASURING DEVICE.

Case OAL and shoulder to end OAL has a certain amount of influence on accuracy, but don't go postal anal...it is only a SMALL amount of influence..."bullet jump" or no jump, powder volume, actuall internal case volume between cases, neck grip and release, and many other variables have more of an influence on accuracy than case OAL.

The only way to know is to shoot about an umpteen million rounds keeping very precise records, comparing ALL the many variables to determine what or if your problem is really an actual problem or a problem in the mind only.

Besides if you don't have a perfectly chambered rifle, perfect barrel, perfectly bedded, perfect EYES, perfect scope with absolutely NO parallax, perfect bullets, cases, exact amount of powder, releasing exactly the same amount of gas, exactly on time...you won't be able to figure out WHICH variable is doing the dirt.

All you can do is your best and not let minutia turn you into a freaking, slobbering, nervous, neurotic twit...I know all about that sort of thing...

Bout the only way I know to get things "perfect" is start with 1000 cases, sort, weigh by volume, amount of spring, neck grip, etc until you find as many cases that spring the same amount, have the same neck tension, end up with the same amount of trim length each time and all shoot into the same hole.

By that time you will have worn yourself out AND your rifle barrel and STILL might not produce small groups because the bullet/powder combo wasn't quit right.

Luck