Just like the title says.

This would be for Lake City .223 brass

It might depend upon the number of cases you have, how many you need and the scatter in the measured weights in your sample.

What is your goal?

I go +/- .5 grain.

So if my target weight is 92 grains that would include everything from 91.5 to 92.5

That is for 223/5.56 brass that I have enough of that I can get that picky and still easily have 50 or 100 round lots to work with.

Last year I made a "lot" of LC brass for out M1A. I was not as picky with it. It's all the same year brass. I'll check my notes and post the weight spread.

ETA: OK. The 7.62x51 brass that I matched for the M1A weighs between 169.6 and 174.5 grains. So this is about +/- 2.5gr or a 5gr total swing.  

This is where the bulk of the brass ended up so I went with it. I've spotted for my son on several occasions and witnessed 5 shot 1" groups with this brass.

I have also shot similar size groups with the optics mounted. The boy does it with the iron sights. Oh to young and have good eyesight eh?

Motor

The scientific method would be to weigh them all, determine the mean and SD, then cull any cases more than +/-1 SD from the mean.

Why?

Because science.

Well... I just blew $400 on a concentricity gauge.

Might as well go full tilt and see if becoming even more anal retentive with case prep and case  selection gets my groups to shrink even more.

This will be out of a 20 inch heavy stainless barreled FrankenAR.

So nothing too fancy just yet

ASSuming it follows a normal distribution...

Assuming weight has an effect on precision.
And that effect is linear.

After about 30-40 cases  you'll say what for. Is your powder going to be the same weight? Is your primer going to have the same flash as the last one? Is your bullet going to weigh the same as all the others? Are they all going to be the same diameters? Are you going to hold the gun the same way with the same tension as the last shot?

Before wasting any effort on this every case must be resized, trimmed to length, primer pocket uniformed and even flash hole deburred. Then and only then will you get a really accurate comparisons. With all other items being identical you have a realistic shot at knowing every case is identical that weighs the same.

For me a 1.0 grain window would seem accurate enough. I would (if I wanted to get this picky) weigh them in batches 91.6 to 92.5 grains 92.6 to 93.5 and 93.6 to 94.5. Or 91.1 to 92.0, 92.1 to 93.0 and 93.1 to 94.0.

The largest pile would get be used for match ammo. The smallest pile used for hunting and blasting ammo and the medium sized pile for practice.

I have messed with this when bored in the winter and although it may have given me a psychological benefit, I doubt anything was accomplished on target when shooting a service rifle in competition. I have loaded unmodified Lake City 7.62x51mm and 5.56mm brass and fired my best scores in tournaments. That tells me that any perceived benefit has nothing to do with reality.

I won't discourage anyone's efforts at improving accuracy, but it's been my experience that better bullets are more effective than uniforming brass. Simply buy Lapua brass and be done with it, after the time you spend doing all this tedious work has to be worth something. For $20 to $30.00 more per 100 cases compared to new USA brand brass it seems worth it to me.

All cases resized the same should have the same external dimensions.

Different weights of these same cases should mean or imply the interal dimensions are different.

Different internal dimensions means different case volumes which in turn means ...or rather should mean different pressures...

Which should mean different velocities...

Which should mean higher or lower impact points on target...which means larger groups.

Or at least that is how I "cyphered" in my head.

I could be wrong

Cool.
Show us. I've not seen a test that showed the difference in brass weight on precision.
Separate your brass into groups you think are appropriate and shoot some 5 round groups, and post your targets.

I am now subscribed to this thread! =)

Googling "weighing brass cases for precision shooting" got me to here:

http://www.sinclairintl.com/GunTech/Weighing-Cases-/detail.htm?lid=16137

Which says:

This pic is from here:

http://precisionrifleblog.com/2012/06/13/remington-223-brass-weight-variation/

Where he used the plastic grid from fluorescent light fixture to do this:

My questions went unanswered.  


I asked those questions and phrased the response because almost always we work with small samples.

Weight variations do not always mean internal volume differences.  The case head has a large fraction of the total case mass and small differences there can have a big effect on overall mass distribution.  

Consider rim thickness, extractor groove depth, case head thickness, variations in primer pocket dimensions, flatness of case head, depth of cut when reaming old crimp, depth of deburring at case mouth,...  many things can cause mass variations that do not show up as internal volume differences (or not as much volume difference as implied by the mass difference).

There is another issue, accuracy in weighing them.  Measure the sample twice and you are unlikely to get the exact same result.  Cases will move from one bin to another.<-- not a big deal, it just happens.

In the end, how many you have and how many you need are key factors in culling cases and what's "acceptable" for use and even "how" one deals with the variation (culling, sorting, sequencing,...).  Whether you do it at all depends upon your goal(s).

If this is just an academic exercise, a full mathematical treatment can be provided.  Why? Because,... maths!

To shoot a target at 100 yards that will qualify for the One MOA All Day challenge.

So 5 groups of 5 shots each, where the group sizes get averaged together and that average has to be under one inch.

This is the best idea!^^^^^^^^^

I did an experiment several years ago to determine just how much effect brass weight has on .223 loads.   I used WW brass (sized, trimmed and deburred, primer pockets uniformed, flash holes deburred, and neck turned) , WSR primers, charges of RL-15 or N-550 powder weighed to 0.1 gr, and 75 gr A-Max bullets.  Using the lightest and heaviest cases (sorted from 1000 once-fired I had on hand), I had two lots of 10 cases with a 3 gr difference in weight.  The average muzzle velocity difference was 16 fps, just a bit more than the 12 fps due to 0.1 gr of powder.  I choose to sort 0.5 gr lots of .223 brass for my long range loads, but the effect will only matter at 800-1000 yards - the vertical displacement on the target from such a small velocity change is negligible at shorter distances.  Unless you control all other sources of variation, the effect of brass weight is negligible.   I also shoot .284, and because the brass is twice as heavy I batch in 1 gr lots.

If you are going to do it, do it right.

Besides resizing them, trim to length, deburr inside and out, uniform primer pocket depth and deburr the flash holes (with a tool that self-regulates depth of cut), then weigh.

If you see a large spread in mass, start checking rim thickness and depth of extractor grooves.  You may find you have multiple production lots causing excessive spread.  

You'll want about 50 cases that are within as tight a group as your sample of cases allows.

I have never seen any benefit from that weighing cases but it also never hurts.

How high up you want to place?  Can you shoot? The Indian becomes more  and more important. Ammo matters, but is no substitute.

What is the gun capable of doing?

MOA by itself is pretty easy.

Oh, look!  A normal distribution (especially given the small sample size).

To whit, why wouldn't it follow a normal distribution?

Quite frankly, look at rifle, bullet, and powder charge weight first.
The challenge can be met with an out of the box Savage 110 in 243 and a Nikon scope.  Powder charge consistency was key.
I haven't been able to get much better than 1.5 moa out of a Ruger M77 in 270.

(Disclaimer, this is not a MOA All day long claim, the rifle belongs to my brother, I don't recall what the load or bullet was, and shooting the 243 was a couple decades ago.  Groups could be covered with a nickel, and I remember trickling the last 0.1 grain into each case.)

I was going to post something similar. I'd think chasing details like case weight would be of more benefit to someone who is already shooting sub MOA consistently and has all the other basics well sorted. I won't say it's a waste of time, but I do think your time and effort would be better spent elsewhere at this point.

This is especially true if you're only shooting at 100 yards- the potential variations in velocity stemming from case weight differences would be pretty much undetectable on target at 100 yards.

Agreed, but it is a hobby and he wants to pursue this - the archer not the arrow; confidence in your ammo and all that.

Regarding distributions, knowing the distribution is nothing in and of itself.  I suppose it is worth knowing IF you find it is bimodal or has other odd characteristics for the sample you measure.  This might imply multiple production lots suggesting further screening and segregation of cases within the sample.

My concentricity gauge  arrived two days ago, so I will see if I can figure out or solve my bullet runout issues first.

EDIT:  just for grins and giggles, I'll have to take a Dremel or my metal cutting bandsaw to a piece of Lake City brass, and cut it open enough to see what the flashhole looks like.

Interiors of flash holes vary widely.  Cutting one tells you almost nothing.  Cutting many is a waste of good brass.  

Get the Sinclair tool and clean them up.  By doing that, you will learn the variability that comes with punched flash holes (as opposed to drilled, as in Lapua).

For my comp bolt guns I batch my brass after a full. Case prep, I would sort my brass to the grain.

Use that brass that batches the most cases, then the rest is used for long range bunny shooting.

My comp rounds are done with more upmost care and attention. My other rounds not so much. As to comp shooting you want to eliminate everything you can. But at the end of the day it's the nut behind the butt that plays the biggest influence on the shot. ( we are the biggest variable)

Just my 2 cents worth on case sorting.

If you're going to go through the trouble, make sure you save a handful from both the heavy end and the lite end.  They're good for reference purposes (i.e. comparing against targets shot with the "good" brass).

I use them as a final confirmation test; if the load shoots adequately with the worst brass I can find, then life is good.

The Army Marksmanship Unit has already done real scientific testing on this.

Here's a writeup about the AMU and what they do with "virgin" Lake City brass

https://www.facebook.com/USAMU1956/posts/10153228155789734:0

Some things I found particularly interesting

Several years ago, we sorted our virgin primed & inspected LC 5.56 brass into 0.3 grain increments. Given large Summer competition demands for LR ammo, we started with 50,000 to 60,000 cases of a given lot. We then winnowed them down into 0.3 gr. lots large enough for the team's Summer needs. (Why, yes, that WAS rather labor-intensive!)
We constantly conduct research to improve accuracy, and to improve efficiency without compromising accuracy or performance. We make no procedure changes without first fully verifying that performance is not degraded. After significant full-distance machine-rest and shoulder-fired testing, we determined that we could safely increase the weight range of LC match brass lots to 1.0 gr. This greatly cut sorting time, while increasing our brass lot sizes. When using some very high quality commercial brass, we weigh a sample of 100-200 cases and, if uniform enough, we can load that lot without weight sorting


Once the brass is ready to load, we set optimum neck tension when sizing. With virgin brass, we use a neck-sizing die with a correct-diameter neck bushing (available in 0.001" increments). We use light spray lube to prevent galling and adjust the die to size the full length of the case neck. Brass varies in hardness (e.g., LC = harder, some commercial brass may be softer.) Thus, for LC brass, we usually size case necks to be expanded 0.003" by the bullet upon seating. Example: if the case neck measures 0.248" after bullet seating, then a 0.245" die bushing is used.)