Easy peasy! 💫
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Once you got some weapons, simply press [Middle Mouse Button] (pressing in your Mouse Wheel) to activate the special MMod feature, like the Silencer for the Pistol and SMG, for example.

The Silencers also makes it possible for you to kill NPCs and Aliens silently!
Also the damage stays the same for using silenced weapons.

The crowbar has a heavier secondary [Right Mouse Button], attack and a very heavy [Middle Mouse Button] attack.

Also there's a laser assisted sniper mode for the revolver, as well as a heavy explosive Projectile you can shoot with the Hive Hand. All activated by the [Middle Mouse Button].