This cheat is on campaign and can be done on any difficulty.

At the start of the level once you've cleared the room pick up a shotgun and head to the part of the level where the machine gun truck is driving around.

Don't destroy it, let it go, then once you move up head to the right until you reach a small car in the middle.

Then turn right and kill the guy on the machinegun and take out the shotgun and hop on the back of the truck.

Aim at the gun then shoot and run if done correctly you should hear crackling and then the truck will explode and gun powder will be around the truck.

PS: it will take two times to finally see the gun powder.