I have developed a pet load with lapua brass, I bought another batch of lapua brass that weights 1.5 grains lighter. Will this effect my groups? Should I just load the same weights of powder as I did with the heavier brass? I hope so!
Yup, I would determine case capacity by weighing them filled with water. Then do the same with some of your previous cartridges for comparison.The real criteria will be the case volume. After shooting some of the new cases you can then weigh 5 of each of the two batches and find their water volume in grains. Now you can get a better idea of how they match up.
I fire form cases 3 times without body sizing for measure of H2Ocapacity. This is also a point where I decide about case trimming/trim length.