Silver weapons is a cheeat unlocked by completing the game on normal, hard, or Black Ops mode. Once the game has been completed on said difficulty, silver weapons is available on that difficulty and any difficulties below it, so completing the game on hard will unlock them on easy and normal as well. The effect of silver weapons is to literally colour your weapons silver, and give them infinite ammunition, although reloading is still needed. There does not appear to be anyway of turning this cheat off once it has been unlocked. A possibly unintended bonus of this cheat is that while your enemies will always have unlimited ammo, their guns are now also silver, making them easier to notice in dark areas.