So your party has been captured by cyberpunk orcs, post-apocalyptic halflings, or some other brand of fucked up sicko and forced to play a friendly game. The losers lose everything, i.e. die, and the winners win it all, i.e. live. We're talking about Russian roulette of course. But how does a GM run this situation? Like this of course!
Spin the cylinder: A player can spin the cylinder an infinite number of times. (However you should punish players trying to "buy time" by abusing this fact.) When they do this they roll a d6 and the result is the number that will kill them when they pull the trigger. For example if Jostalph spins and the result is 6 when he pulls the trigger and the result is 6 he's just coated the wall with a fresh coat of brains.
Pull the trigger: Roll a d6. If the result is your spin the cylinder number you're dead. By default this number is 1 if a player decides not to spin at all.
The d6 is a metaphor for the revolver but this short system can apply for any item capable of randomly killing the player. Playing a friendly game of toss the lit bomb? How about that spooky jar you found that sometimes summons an otherworldly maw to chomp your head off? In that case this system is just what you are looking for!