Without them, any Firearm looks a bit naked. All the available Attachments and their Advantages and Disadvantages can be found here.

Scopes

Reflex Sight

Monocle Sight

Holographic Sight

Combat Scope

Telescopic Sight

Barrels

Silencer

Explosive Muzzle

Trumpet

Stocks

Makeshift Stock

Light Stock

Tactical Stock

Weighted-Stock

Underbarrels

Light Grip

Vertical Grip

Angled-Grip

Magazines

Extended Mag

Drum-Mag

Special

Flashlight

Laser Pointer