I'll probably be smote into oblivion for stating this, but...I would guess, from what I've seen, that the writers weren't really experts on firearms.
Professional Knowledge - Game Design 6 ( RPGs 8 )
Professional Knowledge - Firearms Design 2
I'm not saying that to be a jerk, or to claim that I am an expert...(but I am a professional)...I can say that their nomenclature and many design details relating to the in-game weapons are definitely game-centric, rather than realistically oriented.
Which I am FINE with, by the way - there's also Elves and Trolls and Dragons.
My guess is that they did a bit of research and read that carbines are often chambered for a smaller round than their full-sized counterparts, and extrapolated that into the result you see. Or...just figured they were smaller and therefore shoot smaller bullets.
Or they decided that a SMG-sized weapon firing Rifle damage rounds would make all SMGs obsolete.
Dunno. If I wanted realistic weapon damage, I guess I'd dust off CP2020, again and house-rule in Dragons.
Anyway - my solution was to use the AR rules for weapon modification to build an SBR. Yeah, I use a lot of my modification slots to make the weapon smaller, but I get the power of a rifle in the SMG-sized package.
-Jn-
Ifriti Sophist