Rassilon's roulette was a game of chance played by Time Lords involving the participants to shoot at their head until the gun fired. The survivor was the winner. The gun they used was the Time-Gun of Rassilon.
The Twelfth Doctor later played a game against the Cybock Imperium leader Kronos. Kronos turned the gun on the Doctor, but when fired the gun refused to fire on Time Lord DNA. The gun backfired and erased Kronos from history instead. (COMIC: Gangland)