In my experience, commercial .223 cases and military 5.56x45mm cases have almost identical case capacity and can be used pretty much interchangeably.
However, such a situation does not exist for .308 Win. and 7.62x51mm NATO cases, and for commercial .30-06 compared to military .30-06 cases. In both of these instances, the military cases are 'heavier' with less case capacity, and powder loads must be reduced somewhat in the military cases compared to the commercial ones...
By the way, mek42, where are you located (at least vaguely) in upstate New York? I grew up in Newcomb, New York, in the central Adirondacks...
Forrest