hatcher's work is classic reference. Everything that applied then, still applies today (as the same guns are still around). Same with Kieth, O'Connor, Whelen, Nonte, Skelton, Jordan, Cooper, and the rest of the "old timers".
These gentlemen, in their writings and their exploits are virtually the "founding fathers" of modern shooting sports, and any shooter's library should have at least a few of their works.
and if you want some really complicated math, complicated to me, anyway, check out one of the volumes of "The Machine Gun" (Chinn), there are formulas in there for calculating the weight needed for the bolt of a blowback SMG of a given caliber. Among other things...
Esoteric? sure! There's symbols in those equations I don't even recognise!