Created by the same company who constructed the G9 SMG, the GUARDIAN G12-47X is just as secretive as it's smaller cousin. Weighing in at around 10 pounds, it's standard weight for an assault rifle, but it's cut-down frame puts it into the carbine category. However, the G12 is no slouch, bringing an impressive 600RPM firing rate to the mix.
The 6mm rounds pack more power than other rounds for this type of weapon and are mostly subsonic ammo. The G12 is chambered for this sort of shot as it is intended for stealth missions where sounds can be minimized for maximum effect.
As with the G9, the G12 can be modded extensively to suit any environment, and can fire in the vacuum of space due to lubricants which do not boil away. The G12 pictured above is modified for an Overwatch assignment where distance is valued but a silent takedown is rewarding. With a suppressor, a subsonic discharge is barely louder than the firing mechanism hitting the primer.
Due to the secret nature of the weapon's origins, none of the Commonwealth has access to this weapon.
Actually with subsonic rounds it's a range issue. Generally for every 50 yards, a subsonic round loses an inch on their flight. To counter this, shooters must know hot to compensate for their shot. Just like regular bullet drop but magnified.
You'd be angling this bad boy like an artillery piece!