Just a cpl thoughts on this...and I too have oft heard of the thickness of military brass...that warning is mentioned in Ken Waters "Pet Loads" books as well as "Hatcher's Notebook", and "Handloading" the NRA's excellent book on the subject, by William C. Davis...all authoritative tomes.
The NRA's "Handloading" book, now out of print I think, was published in 1981 and updated in 1986. On page 134, in an article on the effects of component changes, one table collates velocity changes vs. weight differences given identical other loading components. Weights varied from a low of 189 gr for Western, to a high of 214.1 for Peters. Velocities varied from 2570 to 2672 fps. The table also tabulated weight variance within each brand...some as high as 7.0 to 8.0 gr, with high differences in both military and commercial brass.
My experience is with .30-06 and 5.56mm rounds, both military and commercial. From that experience, I can say that military brass does produce add'l velocity, on average, than commercial. Is that due to reduced internal capacity, or to another factor not considered; here are some ideas: flash hole size and regularity, primer pocket thickness, strength of the annealed portion of the shoulder and neck, and brass quality/composition. There are others most probably, that I've overlooked. As to the reason for that add'l velocity, I can't say, other than the fact that it equates to higher pressures.
I will opine that based on NRA's studies, some of it 20+ years old now, that brass can and will differ in thickness, among other characteristics, and can affect pressures. Too, I have no confidence that modern ammunition brass cases are significantly better with quality control than they were 20+ years ago. The old caveat, start well below max and work up slowly still rings true.
A good discussion NWPilgrim...thanks for bringing it up. Best Regards, Rod
Our Flag does not fly because the wind blows against it,
It proudly waves because a soldier's dying breath blows upon it.
USAF Forward Air Controller, 5th Spl Forces,
An Loc, lll Corps, RVN, 69-70, Vietnam Vet '69-'73