Three grains is really not that much. When I get new brass or even once fired brass I go ahead and resize it and trim it to a standard length. Then I weigh it, this includes factory fresh new brass. While Lapua is generally viewed as good to go when I am loading match or target ammunition I still size and trim. Here is a spread sheet I worked up last summer and while it does not include Lapua it does include some GI and all of the brass was once fired and sized and trimmed.
View attachment 777013
The upper left is the start with ten pieces selected at random from each lot. While there is an Average as well as Max and Min weight the key number is the Standard Deviation, that is the number to be looking at and the smaller that number the better as it will reflect the best uniformity in weight across the board. The Federal was hands down the tighest followed by the WCC 10 GI brass which has shot extremely well for me. Still have those 50 cases in order and this summer plan to finish by shooting them. I have some new Lapua .308 in standard and Palma (small primer cup) I hope to get weighed and measured.
Anyway if you want to try something interesting weigh a 10 case sampling after you size and trim and then weigh each listing a Min and Max weight, Average Weight and the Standard Deviation. While I let Excel do the math the standard deviation can be calculated:
To calculate the standard deviation of those numbers:
- Work out the Mean (the simple average of the numbers)
- Then for each number: subtract the Mean and square the result.
- Then work out the mean of those squared differences.
- Take the square root of that and we are done!
Ron
Now if it would quit snowing and freezing I want to load those 50 cases and see what I get over the chronograph and how they group.
