Are you absolutly sure that you bought all the guns? Here is a full list of the weapons that are needed fo rthe trophy. Go through the list and make sure that you aren't missing any.
M92F
H&K P8
SIG P226
M93R*
Ithaca M37
M3
Jail Breaker
Hydra*
VZ61
H&K MP5
AK-74
SIG 556
S75
Dragunov SVD
H&K PSG-1
S&W M29
L. Hawk
S&W M500*
Hand Grenade
Flash Grenade
Incendiary Grenade
Proximity Bomb
Grenade Launcher
Rocket Launcher
Stun Rod
Gatling Gun*
Longbow*
Rotten Egg (Needed if you have the latest version of the game or if you have the Gold Edition of the game.)
* = Can not be purchased until a specific weapon is fully upgraded.