As a primary weapon :

Automatic weapon

+15% movement speed

+400% clip size ( hidden stat )

+650% primary ammo ( hidden stat )

+10% switch speed

+25% more accurate

Aim-down sights

Mini-criticals on headshot while aiming-down sights

Slowed down while aiming-down sights

+5% more accurate while aiming-down sights ( hidden stat )

-10% firing speed while aiming-down sights

-15% damage penalty

-20% jump height