How much weight variation should be in Lapua 308 brass?

[CatShooter]

I personally believe that it's possible that weight differences can indicate volume differences but not necessarily so. Correlation is not causation.

If you want to sort by case volume, you need to measure case volume, not weight. If somebody has conclusive evidence to the contrary, I'd be glad to be proven wrong; weighing a large number of cases is relatively easy, measuring volume on same is not.

Not to say there isn't value in weight sorting as a means to find truly irregular cases. I just weight sorted 200 pieces of Lapua 6.5x47 brass. Out of 200 there were exactly 5 cases that were more than +/- .5 grain off the mean.

-nosualc

The biggest problem with sorting by volume is the case body itself, is never a uniform hardness, so even if you fire 100 or 200 cases, 3 times each, the amount of body spring back of all the cases is not uniform, and if the hardness is not the same, then the volumes are not the sasme.

So to measure the internal volume, the case must be constrained in something that will force it to conform to a standard size and still give you access to the mouth.

You can run a case up into a file/trim die, and then fill it with water - that will give you the most accurate measurement.

But that is more effort than I am willing to do over this issue.

Jlow's chart is very telling. It works for me.

 

[jlow]

Correct!

If a person go by what QL suggest which is to do volume measurements after firing and before FL sizing, well, the diameter of the neck will now vary based on the charge weight and certainly based on how case harden the neck is due to differences in spring back. Not saying QL is incorrect as the program is asking for an approximation of case total volume.

However, when we use the same conditions to compare differences in case volume, then these small volume differences can easily be muddied by variable such as differences in case ID (body, neck) due to differences in charge weight and spring back.

This is why if you are really only interested in finding out if differences in case weight translates to differences in volume, you either use new unfired brass that Lapua has sized for your, or you must FL resize the case, the neck, and make sure that the neck has a consistent degree of spring back by annealing.

The lesson here is even though case volume is measured in both instances, they are for different purposes and one method does not work well for both.

 

[jlow]

Problem of weight variation solved!

Brought another 300 cases and weight sorted all 500 in 0.5 grain increment as in the photo. Now I got enough in about 3 batches where weight variation is around 1 grain. Notice the sharp demarcation in the two populations i.e. almost no cases with weight between 172.50 grain and 172.99 grain…

 

[chkunz]

Problem of weight variation solved!

Brought another 300 cases and weight sorted all 500 in 0.5 grain increment as in the photo. Now I got enough in about 3 batches where weight variation is around 1 grain. Notice the sharp demarcation in the two populations i.e. almost no cases with weight between 172.50 grain and 172.99 grain…

Our work here is done! No not really. Even though most folks that post and respond on this forum bad mouth those that utilize statistics probably because they do not understand the discipline, I think it is clear that you are dealing with two different populations. I also like Lapua brass and pay extra for the quality but this does not meet their reputation on quality. No one has shown what this is worth on the target but we pay for consistency so we do not have to worry about it. You did not get the quality that you paid for.

 

[BCoates]

Problem of weight variation solved!

Brought another 300 cases and weight sorted all 500 in 0.5 grain increment as in the photo. Now I got enough in about 3 batches where weight variation is around 1 grain. Notice the sharp demarcation in the two populations i.e. almost no cases with weight between 172.50 grain and 172.99 grain…

So which group has the most consistent case neck thickness? It would be interesting to see if some weight groups are more consistent than others.

 

[jlow]

T-Rex – I have to say I agree somewhat.

First, It is absolutely clear that there is two distinct population. However, to be completely honest, it is hard to judge whether I got the quality I paid for as for me to determine that, I would need to know the cost of brass that is truly one population in weight/volume and I don’t have that number.

Second, I frankly find it really odd that their brass consistently have two distinct populations that they must be completely aware of but does not appear to make any effort to separate? I mean the whole idea of buying Lapua brass is to get consistency, they do really well on the neck thickness variation but case volume consistency would of course be yet another important variable people are interested in.

Of course, this does means that we have to buy a large batch to get enough of a consistent batches so perhaps not such a bad marketing scheme…. :

BCoates – I have not measured and so don’t know the answer. Will be doing some studies with samples from this pretty large batch of brass perhaps the next week. Mostly looking at weight vs. variation but I can check neck thickness variation.

 

[jlow]

Did the weight vs. volume determination today with 40 cases (5 from each weight group that are 0.5 grain difference in weight) with the rubbing alcohol method. Got good correlation (see graph) of 0.83.

One interesting thing I found was I was hoping to use the 2 really heavy cases that I reported in the OP i.e. those two that were in the 177 grain range. Found them, weight one and was going to insert the 21 Century primer pocket plug but found that I could not. Turned the case over and lo and behold both cases were PRIMED from the factory! Here is the picture to prove it! Well that is OK by me as that means the whole lot had an extreme spread of about 3.5 grain instead of 6 grains! ;D

 

[Area Man]

Here's the distribution of a box of 50 243 brass I got from Norma. They said it was presorted by weight. Looks like it was but then 2 lots were put in the same box.

 

[jlow]

LOL! Nice! ;D

 

[memilanuk]

Since we're all in the sharing mood...

Batch #1 - ~250 pcs of Lapua .308 Palma brass, 3x fired

Batch #2 - ~500 pcs of Lapua .308 Palma brass, 2x fired

I was thinking about keeping track of the change in weight distribution over time - weigh again after 4 firings, after 5, etc. to see if the curves flatten out at all.