I don't know, man.
I think, for example, an AR15 chambered in 9mm has the same felt recoil as a 5.56 or 5.45. Mostly
because it's a blow back design. Even worse, honestly, with some Egyptian 9mm machinegun ammo
(corrosive) kinda on the hot side.
Ya know, even the MP5 is harsh for the size of the cartridge. But then again, what HKs don't.
What kinda carbine were you shooting?