6MM PRC

[ELR LVR]

I don't see how usable case capacity measurably changes when necking up or down, as you're not altering the body.

true, and the differences arent much
but the measurements taken and reported of capacity must be going slightly into the neck
if they were not then you would be absolutely correct
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Example: Measured in Cubic Inches
300 PRC - 0.417787
7 PRC --- 0.359592
6.5 PRC - 0.307813

Chart of All Rifle Cartridges by Case Capacity – Backfire

backfire.tv

 

[ELR LVR]

Seems every manufactures brass even in the same cartridge has a slightly different amount of case capacity.
I've only measured case capacity in Hornady and ADG 6.5 PRC brass but they were both right at 68gr H2O with Hornady having a couple tenths more. I don't have any Lapua but they're usually pretty close to whatever Hornady's is.

I use Lapua in both my 6-284 and 6.5-284. I'll have to look back through my notes but if I recall it's right around 67gr H2O and that is from a fired case. New brass and sized brass always measure slightly less then fired unsized.

Yes it also depends on the powder such as Ball vs Stick
with ball and a drop tube I can use 67 grns in the 6x284

 

[lancexxx]

Thanks for all the info

 

[Woollybugger]

When you build a cartridge in GRT it does a calculation for capacity in grains of H2O. It is often conservative when compared to the fired cases I have measured. What is most interesting to me is that when seating depth is adjusted in GRT the capacity is adjusted and pressures rise in their calculation. The deeper you seat the less effective case volume and pressures and velocity rise. The load I have been playing with in a 22 GT (77 noslers and Re 17) has matched really well with what I have modeled in GRT.