Enter the "Complete Current Level With Silent Assassin Rating" cheat on the training level and you will unlock every weapon.

Unlock Gold Desert Eagles DUAL
On the first level (asylum Aftermath) make sure you have the keycard described in the minigun method. You get the keycard on mission 9 (hidden within the mansion). Go to the part of the level where there is a ramp going down with locked double doors (which you cannot normally go in) and the door which you have no option but to take (or you can climb through the window) You can now actually get into this locked door and on the floor in front of you will be dual Desert Eagles, GOLDEN EDITIONS!