AFAIK the difference between the two popular cartridges is penetration vs energy absorption. Is there any difference? At short range (Pistol) where I can point shoot – I want a slow, fat, heavy bullet that doesn’t go on forever. At medium range (SMG) with I want balanced faster bullet with lower recoil. At longer range I want a faster, longer, thinner but still heavy bullet(Rifle). At longest range I want an aerodynamic bullet with as much mass as possible(Sniper or Vehicle Mounted rifle/MG) As far as I know 45/50 pistol 9mm SMG, .65/.7 Rifle, and .5BMG are pretty much the optimums. And if you’re using two bullets you are firing on a borderline between pistol and smg. So most caliber talk that I know of consists of trying to produce the wrong effect from the wrong size of weapon. Now, I can totally understand the 9mm affectionados. Because at double the rounds and longer distance using sights it’s bridging that distance between a pistol and an smg. A 1911 in 9mm (The original caliber believe it or not) has fewer rounds, but makes a great race gun. A 6″ 1911 with the extended sight picture is probably even better. And so I think the science says and now the logic as well both say that the data is in enough, and that it’s possible to buy ammunition that negates the differences. But if we’re talking about military grade (volume) ammo, I think the story is finished. It’s basic physics. A fat heavy thing transfers more energy faster. Thinner lighter thing more slowly. It’s just resistance. simple stuff.