GURPS Old West - leaving the hammer down on an empty chamber

[Kale]

Modern revolvers have a feature in the striking mechanism to prevent this. Ye Olden Day revolvers, however, that hammer is sitting right against the firing pin. Any sharp blow to that hammer get's transmitted through the pin right into the primer on the back of the bullet. Basically use common sense; anything that's likely to cause the hammer to be struck will have a chance of setting off the bullet.