Lapua 308 brass weight distribution (pic)

[ktgross]

I just picked up 400 new Lapua 308 cases and decided to kill some time. I sorted them on my digital scale that measures to .0001 grams. I sorted them in .002 gram increments. The lightest weight starting in the top right of the pic and going left, then from bottom right to the left, ending with the heaviest. Trowing out the one far left and one far right brass, the weights ranged from 11.176 gms (172.5 gr) and 11.324 gms (174.8 gr). The shape of the curve is kinda weird though.

 

[Ty Linger]

Re: Lapua 308 brass weight distribution (pic)

Are you going to be able to use all of the brass, if not, how many will you discard in order to keep uniformity?

 

[ktgross]

Re: Lapua 308 brass weight distribution (pic)

I've sorted them into 4 groups and will sell 2 of the groups and keep the middle 2.

 

[kombayotch]

Re: Lapua 308 brass weight distribution (pic)

Did you sort after doing case prep (i.e. uniforming)? I assume you are doing it if you're that concerned about uniformity...

I would keep the outliers for foulers, load development and/or tool setup. Basically, anything that is not critical or could harm the "golden" cases.

 

[9H_Cracka]

Re: Lapua 308 brass weight distribution (pic)

It looks right to me, when you understand that the brass in one box comes off of two machines.

After sorting 1000 of them, 500 from one "lot" and 500 from a different "lot" (I use that term in quotes because it really is 2 lots per lot if you will) it is clear that in each box within the lot 2 different weights are present, generally separated by on average 1.5 grains (the median in statistical terms)

 

[Rob01]

Re: Lapua 308 brass weight distribution (pic)

You shooting bench rest matches? If not put them in a pile, prime them and load them. You won't see a difference.

Also did you measure them and trim them to the same length before weighing them all? If not then differences in length is going to add or subtract weight and throw off your chart. I have had Lapua from 2.005 up to 2.010 in length.

 

[yuri]

Re: Lapua 308 brass weight distribution (pic)

I did something similar out of curiosity. .308 lapua unpreped brass spread was from about 11.080gm to just over 11.210gm on mxx-123. Interestingly weight spread increased after brass prep. (primer pocket unif., trimming to uniform length, neck turning). I seriously doubt any practical benefit of weight sorting brass.

 

[steve marshall]

Re: Lapua 308 brass weight distribution (pic)

You should definitely trim before you weight sort. Why that .010" variation in length could easily introduce a 2 or 3 tenths variation in weight.

 

[Jedi]

Re: Lapua 308 brass weight distribution (pic)

Futile exercise

Case weight means nowt, unless you have away to determine weight distribution in case vs wetcap

Wetcap is only number that should be of concern as it directly affects load performance

I use average of averages
ie
weigh 50 cases, measure wet cap, calc average wet cap
case weight average - wetcap average = case cap (H20) average
punch info into QL .... done deal

 

[yuri]

Re: Lapua 308 brass weight distribution (pic)

I am no expert in this, but it seems to me that weight or inside volume sorting is futile, at least for .308 Lapua brass. It does make sense to have brass uniform in shape and weight. In all likelihood, making Lapua brass more uniform even if it was possible at hobby-reloading level would not affect accuracy . I noticed Lapua .338 brass weight variation to be about 1.2%. That is pretty good! It is comparable to powder and bullet variation. I never measured case volume variation. It seems that internal case volume is better parameter to follow than weight,just like Karl2U noted. For internal case volume to be meaningful it should be measured after the brass is fully fire-formed to the chamber. Karl2U, what kind of volume variation are you getting? How do you measure it and how do you deal with the primer pocket? What I am trying to say is that it is difficult to improve this variable.
Assuming one made perfect brass- it certainly would not hurt anything-but will it help? I doubt it. Brass uniformity in terms of its weight/internal volume is already good, trying to improve it is not beneficial until all other more and equally important variables are taken care of. All of this is pure speculation on my part. I will appreciate any pointers to any investigations of this issue.

 

[GuinnessNM]

Re: Lapua 308 brass weight distribution (pic)

It would be interesting if ktg could sort the brass into 3 groups (lower left peak, lower right peak, upper group) load them up and report if any change in the point of aim can be seen. I expect not.

The volume of 2gr of brass is about .015 cm3. Lets assume all of this volume takes away from the case volume. The volume of a 308 case is about 4 cm3, and the combustion volume at peak pressure is about 10 cm3. So, if I got my numbers right, the weight variation represent a few tenths of a percent variation in the average combustion volume with correspondingly small effect on the peak pressure. I expect the effect on group size will be unobservable compared to other effects.

 

[Grump]

Re: Lapua 308 brass weight distribution (pic)

Send me the funding and I'll gladly systematically test these variables.

 

[GuinnessNM]

Re: Lapua 308 brass weight distribution (pic)

Case weight may be a more sensitive than water capacity as a measure of the initial combusion volume, since the density of brass is 8.5x higher than water and the water has problems of meniscus and air bubbles. But there are assumptions involved in using either as an indicator of the combustion volume which are hard to test. So far as I can find, nobody has reported any observable correlation of cold case weight or volume with group size or centroid for good quality brass.

 

[yuri]

Re: Lapua 308 brass weight distribution (pic)

I agree with you, it will be impossible to notice any difference; even if there is one it will be obscured by the host of many other variables, much-much more influential than brass weight variation. There is no need to waste time weight-sorting brass.

 

[senna.bra]

Re: Lapua 308 brass weight distribution (pic)

Futile exercise

+1

 

[79M1a-texas]

Re: Lapua 308 brass weight distribution (pic)

"it all depends on what you want to do". To the group that states it a futile exercise to sort by weight, the question might be what application are you making the reloads for - 100 yard range work; hunting loads; F-class competitions or bench rest?

If sorting by weight has no purpose, then how many reloaders mix their brass (Fed, Rem, Nosler, LC, Lapua) into one big pile and then just start dumping powder?

 

[MontanaMarine]

Re: Lapua 308 brass weight distribution (pic)

2.0gr spread on brass weight is nothing to worry about for tactical accuracy.

I sorted a bunch of milsurp out. It was a 8.0 gr spread. At 600 yards, the difference in group size between sorted and unsorted was barely perceptible. The sorted group was 10 rounds with brass 1.0gr consistent. The unsorted was one of each grain weight, 175-183gr, and a couple more tossed in to make an even 10.

 

[robpiat]

Re: Lapua 308 brass weight distribution (pic)

There is some real differences in surplus brass. I think you are in the land of negligible on all that. Water volume would be another way to take it a step further.

 

[GuinnessNM]

Re: Lapua 308 brass weight distribution (pic)

I would not mix brass of different manufactures or head stamp simply because it is too easy to sort these. It is well known that one must load differently for Lapua vs. Winchester brass to get the same velocity. Might as well do what is easy to do to make the ammo consistent, at least within one box.

It is more effort to weigh brass or measure water volume - enough for me to consider whether it is worth the effort. And it is questionable what is learned if the measurement is not done right. In another thread I once attempted to estimate the effect of small volume changes correlated with case weight. I think my conclusion was that 2gr variations would be unobservable at the 1/2% level for trimmed brass of the same manufacture. 1/2% is the velocity standard deviation of Black Hills match ammo. Good factory brass is more consistent that that. I do not sort my new Winchester brass and I am able to reproduce the BHA velocity and SD.

It would be interesting to see some real data.

 

[esromvre]

Re: Lapua 308 brass weight distribution (pic)

weigh all you want and the ones that you don't want can come to me.

I really will have to suffer with over and under weights.

what I mean is I will take the hit for the team and use them...lol

 

[Sumpter Steve]

Re: Lapua 308 brass weight distribution (pic)

I'm not a good enough shooter to worry about case weight. I would trim them to length, load them up and burn'em.

 

[TomS308]

Re: Lapua 308 brass weight distribution (pic)

Weighing brass is a waste of time. It makes no difference on the target. The only time weighing brass is usefull is.. when dealing with maximum loads.. and switching manufacturers of brass, or using much heavier military brass.. Thats the only time you may want to compare brass weights, and reduce loads with heavy brass. i`m not talking about a few grains difference.. but maybe 20 grains difference. Also, when sorting brass.. you dont go by grains.. you go by percent of difference. example, small, medium cases.. 1.5 % large cases, 3.0 % of the average weight. Also, once you prep all the brass, trim, neck turn to uniform, flash hole deburr.. all your weights change again.

 

[tarzan]

Re: Lapua 308 brass weight distribution (pic)

may not help but if it helps any matches are sometimes won by 1 point where 1/16 of a inch would have touched the line

 

[Gruff]

Re: Lapua 308 brass weight distribution (pic)

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Rob01</div><div class="ubbcode-body">You shooting bench rest matches? If not put them in a pile, prime them and load them. You won't see a difference.

</div></div>

According to Fred Sinclair, brass should be sorted to within 2% of case weight. With 175 gr cases, you've got a 3.5 grain window to play with. Sorting Lapua brass is a waste of time that could be better spent shooting.