Suppressor setting is usually there to allow firing full powered rounds through the suppressor: it is used to moderate the gas entering the operating system in order to slow down the moving parts to normal speeds. Suppressors increase the amount and pressure of the gas used to cycle the weapon.
AK will normally operate with suppressor and different types of ammunition (in my experience), other weapons not so readily. Also, even though I'm sure there is such ammo, but sub-sonic 5.56 NATO is not really worth it. It is not much else than a .22lr.
Subsonic rounds for high velocity cartridges are a niche market. Subsonic 5.56mm has very little use since it lacks power, penetration and range. I've been told performance is worse than an MP5SD, while not being any more silent. If you want somewhat improved penetration over the 9mm round you may opt for a P90 with silencer and SB193 ammunition. Alternatives like .338 Whisper in AR-15 type rifles have been developed, but were never adopted in quantity in LE or military use.
For 7.62x51mm the story is somewhat different. Military subsonic rounds are less numerous than civilian, but Lapua has subsonic 7.62mm and MEN as well.
Most battle rifles and sniper rifles with silencer are used with full power rounds. Subsonic rounds are mainly used in the CT role at ranges of 100 to 400 m. If subsonic round are used it is often in alternatives with integrated silencer. Those function well with both supersonic and subsonic ammunition. Examples are AWS, Ultima Ratio, DSR-1, APR, etc.
I know of MSG90 and Galil Sniper/SR-99 semi automatic sniper rifles being used with subsonic ammunition. AFAIK even G3-K cycles well with subsonic ammunition.
A notable round that is subsonic, armour piercing and retains punch is the Russian 9x39mm. Its range is less than 7.62x51mm subsonic, accuracy might be somewhat lower as well. The AS Val and VSS are reported to be quality weapons with limited noise. The SR-3M and 9A91 focus more on the power of the 9x39mm than the noise reduction.
Last edited by Magister; 07-17-2012 at 07:44 AM.