[Unclenick]

As near as I can tell, these days only the .308/7.62 has significant case capacity differences between commercial and military. The new, never fired LC cases I have all present a very slightly lower weight and higher capacity than the Winchester .223 cases I have (all about 12 years old, now).

The factor that affects peak pressure is the capacity in the case when it is swollen out against the chamber under pressure. The chamber constrains all cases to close to the same exterior dimensions (except at the head, so double-check your lot in that regard). As a result, the weight of the case is a pretty good indicator of comparative water capacity it will have under the bullet. I find typically that each 1.5 grains less case weight indicates the need for about 0.1 grains more powder to keep performance identical. With some powders it's a little more or a little less, but the bottom line is that if two cases have less than about 4 grains brass weight difference, the chance of seeing the effect on your target, even without adjusting the load, isn't great.