"Obtain all available weapons."
There are two ways to earn weapons in the game. First, you can find silver cases throughout the game and pick them up, or you will unlock extra weapons by upgrading existing weapons. If you are playing co-op and your partner picks up a weapon then it will then be available for you to buy, or you can just go back into the level and pick it up for yourself later.
You must have one each of the following in your inventory at the same time.
Hand Gun:
M92F - Found in Chapter 1-1
H&K P8
SIG P226
M93R - Fully Upgrade M92F
Shotgun:
Ithaca M37 - Found in Chapter 1-2
M3 - Found in Chapter 3-3
Jail Breaker - Found in Chapter 6-1
Hydra - Fully Upgrade Ithaca M37
Machine Gun:
ZV61
AK-74 - Found in Chapter 5-1
H&K MP5 - Found in Chapter 2-1
SIG 556 - Found in Chapter 5-2
Rifle:
S75 - Found in Chapter 2-1
Dragunov SVD
H&K PSG-1 - Found in Chapter 5-3
Magnum:
S&W M29 - Found in Chapter 3-1
L.Hawk
S&W M500 - Fully Upgrade S&W M29
Misc:
Grenade Launcher - Found in Chapter 4-1
Rocket Launcher - Found in Chapter 3-1
Stun Rod
Gatling Gun - Fully Upgrade ZV61
Longbow - Fully Upgrade S75
Frag Grenade
Flash Grenade
Incendiary Grenade
Proximity Mine