" Does the weight of brass case have a direct relationship to its volume, and if so can the case weight be used to batch cases by volume"

This is an important question because case volume is an important factor in the safety, velocity and accuracy of hand loading. The problem is that measuring case volume is tedious and prone to inaccuracies.

I set out to eliminate some the tedium whilst looking for a relationship.

I didn't set out to actually measure the volume of the cases I used. The data given aren't measures of case volume, please don't tell me the data are wrong and Lapua cases are bigger than that, the volumes I measured are not the volumes of each entire case, the reasons will be come apparent.

The equipment used: 30 x Lapua .308 three times fired in the same rifle. Cases are neck turned, trimmed to 51.1mm, have been de-primed, primer pockets cleaned, and neck sized with a Wilson bushing die. An A&D FX500i scale set to weigh in grains (gn). Resolution is 0.02 gn. I used water heated to 25 deg C initially that measured 22 deg C at the conclusion (I heated the water because I would have my hands in the water for an extended period and its cold in Tas) The change in density of the water was 0.08% over the time and is ignored.

Method and comments

I selected 30 cases randomly from a cohort of 200 because 30 is the normally the minimum statistically significant sample size.

I wanted to eliminate the influence of the meniscus on the results so I devised a simple stand that enabled the case be weighed neck down on the scale. It was simply a 50c piece with a 144gn ADI projectile glued to it.

I placed each case individually into the tub of water, swished it horizontally 4-5 times in an attempt to eliminate bubbles and then fitted the stand into the case mouth. I removed the case from the water neck down and dried it using a super absorbent cloth. Most and sometimes all of the visible water beaded off the case and the cloth easily wicked any remaining water away.

At this stage the primer pocket was still full (if it wasn't full the water was tipped out and the process restarted) and this water was removed with a cotton bud. Meniscus basically eliminated.

The seal between the projectile and case was very good and no overt force was used to seat the projectile into the case neck. After a minute or so in some cases a bead of water would leak but it didn't influence the results because it was still weighed with the case. see photo.

Now I realise ( as stated above) that I didn't weigh the entire volume of water that would fit into each case, but the volume not measured due to space occupied by the projectile is the same in each case, because all necks were resized using the same equipment, any variation I am treating as minimal and ignoring and in any case is much less than possible variations in a meniscus.

The data was entered in a spread sheet and the net weights calculated, graph produced and R

^{2}value calculated.

Results

The results are given below in the table and graph.

Note that if you look along the 48.0 gr line at the bottom of the graph, almost the entire range of case weights are accommodated. This means that cases weighing in the range of 172 ~ 175 gn the water capacity can be approximately the same

The Pearson correlation coefficient R

^{2}value for the data is 0.

.

Conclusion

The R

^{2}value is zero. This means that there is NO relationship between case weight and case volume, at least for this batch of .308 Lapua large primer brass.

So weighing brass is a waste of time in terms of inferring the case volume.

The data also show the irrespective of weight, that the maximum volume variation was 1.04gn and 0.74 gn with one outlier omitted. Is the close enough? If so, just use the brass as is.

It is possible that there is some tolerance stacking in some of the data but I am confident (unless I have made a huge error in logic) they are as accurate as can be. These factors could be, change in water density and variation in projectile seating depth.

Pete