Suppressed ARs are generally SBR setups designed for building clearing, hostage rescue, etc., so they are typically designed for use under 100 meters. A 55 grain bullet leaving the muzzle at 1000 fps would be pretty effective at 50 meters, and still dangerous at 100 meters, but pretty much useless at 250 meters. As far as gas systems go, there are ways around the reduced volumes and pressures involved, such as opening the gas port area, or even suing multiple ports to feed the system. It would be important to never fire full power ammo in such a gun, though, as it would be easy to over accelerate the bolt.
223 tactical 55 gr., .209 B.C.
www.hornady.comRange (yards)
.............. . Muzzle 50 100 200 300 400 500
Velocity
(fps)1000 955 917 852 798 749 705
Energy
(ft.-lb.)122 111 103 89 78 69 61
Trajectory
(25 yd. z) -1.5 -0.8 -9.6 -58.9-155.9-307.0-519.6
Come Up in MOA-1.51.59.128.149.673.399.2
the bullet still would have more energy than a .22LR at 300 yards, but you'd have to aim almost 14 feet high....