So...if you spend much time here, reading tons of personal accounts and people's combined experiences and wisdom, you'll see a pretty clear trend and pattern, along with a lot of personal evolutions that didn't start this way:
1) The more someone shoots, especially along a dedicated training path, the less cartridge-choice matters
2) The less recoil a gun-cartridge combo has, the more someone will shoot and practice with it
3) The cheaper the ammo, the more someone shoots & practices
4) The more someone shoots, especially in field-realistic training scenarios, the more they realize lower-recoiling setups just shoot better for them, and the more they want to shoot
5) Everything in North America has been killed with .223 and 6mm bullets, but tipped "match" bullets seem to be the most effective
6) The more you like your gun, the more you'll shoot with it.
Bottom line: if you have a new gun in mind that really rings your bell, get one in a chambering that's cheap to feed it and lower-recoiling, and shoot the hell out of it. Because you'll enjoy it, and be better for it.ju