Euston has the exact same damage profile as the CARB-9, as well as the same RoF, but has (no joke) 10x less recoil. If you are a medium and are currently using the CARB-9 as your primary, upgrade to the Euston for way less recoil (and a little less spare ammo).
Carb-9 Stats...
Leg Shot Damage: 17
Torso Shot Damage: 23
Head Shot Damage: 34
Rate of Fire: 909 rounds per minute
Reload Speed: 1.950
Standing accuracy: 2.3
Crouching accuracy: 2.1
Sighted accuracy: 2.0
Euston Stats...
Leg Shot Damage: 18
Torso Shot Damage: 25
Head Shot Damage: 37
Rate of Fire: 909 rounds per minute
Reload Speed: 1.650
Standing accuracy: 1.2
Crouching accuracy: 1.0
Sighted accuracy: 0.8
Lower accuracy numbers are better (lower spread per shot). The euston is pretty much better in every way than the carb-9 according to those stats... more damage, same fire rate, faster reload speed, better accuracy/stability, and there's the fact that it can attach an underslung grenade launcher. My only issue with it is that the euston has significantly less ammunition, which makes it riskier to use as a non-soldier character (soldiers never really have to worry about ammo, other classes do).