Certain weapons have a silent mode - it can be added to others as a special accessory at the cost of an item schtick. When firing in silent mode, a weapon has no burst signature and makes very little sound.
A weapon retrofitted with such a modification takes a -2 damage penalty when firing in Silent Mode. It takes a 1-shot action to switch modes - no time if you have LightingReload.
This modification not apply to Launchers, GyroJet Weapons, or NeedleWeapons.