Before starting a regular game, start Mercenaries once. No matter whom you will choose and how long you will last, providing you will earn at least one $. Now, quit the game and edit the file bio3.ini in its directory. You should see a following text at the end of the file:
After this goes a 20 characters long digit and letter seqence. Now, replace everything with the following:
If you did everything correctly, you should now have Assault Rifle, Gatling and Rocket Launcher with unlimited ammo in the Resident Evil game (plus additional $4150 on the Mercenaries account in the case you wanted to try and compete for the one last bonus).