And I would tend to agree with you. I mean, there are other guns where the names just don't seem to match up, like the Drognav that looks like a DSR. Is it even possible to have a rifle round in a straight clip like that? Without a rifle round then it wouldn't be a carbine, nor an M4, I agree.
It's definitely possible to have large rifle rounds in straight magazines, I was just basing my speculations on the fact that the Carb-9's magazine looks too
thin for rounds comparable to the 5.56. I realize that there is a possibility that when playing Brink, weapon descriptions won't even mention the type of ammunition that they use, but for the purposes of fulfilling combat roles, the looks of the Carb-9 tend to lend itself towards a shorter-range weapon, as far as realism goes. Again, I have no idea to what extent the "what it looks like" to "what type of gun it is" correlation will play out in the actual game.
On the Brink Wiki http://brink.wikia.com/wiki/Weapons chart, the weapon in question is stated as the "Carb-9 SMG." So, as Shadowcat stated, the name of the weapon is a bit misleading. 9mm carbines
do exist, but again; purely semantics. On the whole, I don't think it really matters - I still rather like how the weapon looks.
On a different note, if both versions of the SCAR-L and SCAR-H are in the game, I theorize that they will probably be in the same weapon group, but they will have different stats. Since the SCAR-H is chambered for 7.62 (if I remember correctly, correct me if I'm wrong) it will likely have more recoil, greater damage, and less ammo capacity than the 5.56-oriented SCAR-L.