Arsenal (pg. 29)
MACHINE GUNS AND CANNONS
All machine guns and assault cannons can be equipped with
top-, barrel-, and underbarrel-mounted accessories, unless otherwise
noted. They cannot use silencers or sound suppressors and
suffer from double recoil modifiers (see Recoil, p. 152, SR4A).
Firing these weapons requires the Heavy Weapons skill.
So no silenced Guass Rifles or suppressed MGs.
Of course you can always take a barrett, slap a large Firing Mod: Full-Auto on it, pop Electronic Firing in it and a Sound Supressor, Shock Pad, and Gyro Stabilization Harness on it for a nice anti-armored troll weapon.