Army of Two: The 40th Day

You will unlock this achievement on your way to getting "All Guns Blazing"
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This achievement requires you to score a kill with one weapon (either the primary, or secondary) of every kit in versus, excluding the grenade launcher and RPG. Start by using Assault Kit 1, score a few kills with a weapon (it may seem a bit confusing at first, but you only need to down them, then ensure they die by some way). After you've gotten a few kills with a weapon in the kit, simply switch kits and repeat the process.

LuckyConquerer2 has noted that the rocket launcher may be required in order for this achievement to unlock properly.

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Kits:

~ Assualt Kit 1
SCAR-L Assault Rifle
G18C Pistol

~ Assault Kit 2
Felin-2C Assault Rifle (w/scope)
TD-V .45 SMG

~ Close Combat Kit 1
AA-12 Shotgun
MP5 SMG

~ Close Combat Kit 2
R870 Shotgun
M-416 Assault Rifle

~ Sniper Kit 1
VSS Sniper Rifle
TD-V .45 SMG (repeat)

~ Sniper Kit 2
M107 .50cal Sniper Rilfe
Type 77 Pistol

~ Commando Kit 1
AK-47 Assault Rifle
DE-0 .44 Pistol

~ Commando Kit 2
G36 Assault Rifle (w/silencer)
Type 77 Pistol (w/silencer)(repeat)