Sounds like what you are talking about is an inhalant called amyl Nitrite, a short lasting drug. Amyl, often called poppers can be broken up and inhaled to give a person a quick dose of muscle relaxant and euphoria. Many people often use these to enhance sexual pleasure also. These can be seen in media like, Fear and Loathing in las vegas, which is used both for pleasure, and as a prescription for unstable angina (heart problems). Also, you may be thinking of Nitrous Oxide, which comes in a metal "bullet" which is another inhalant that causes euphoria and relaxation. This gas is also used as a whipped cream proppellant. Hope this helps... Well, I'm guessing here.

If I was to put a "drug" in a silver buller case, it would be Silver Nitrate. It is a liquid.

When exposed to the skin it turns skin black and can't be washed off. Your skin eventually sheds the black off.

You don't want it on your skin however, as it is toxic.

In the fantasy world, it is put into bullets to kill werewolves.

I really can't see anyone bothering with that, as in the real world

most rifles or shotguns could fix that problem.