Assault Rifle: Pistol. Quick to take out, which is useful for when I need to finish somebody off but I don't have time to reload.
SMG: Machine Pistol to continue spraying when my SMG's mag is out of ammo, but I still need to take somebody out.
LMG: Machine Pistol because they can quickly take out anyone that gets close to me while I'm trying to get to a safe spot to reload.
Sniper: Shotgun because they cover the area sniper rifles are weakest at.
3-round burst and FAL: Shotgun. See above.
Riot Shield: Shotgun: The Shotgun lets you quickly take out enemies that are getting a little too close.