For Russian Roulette, the deadly game is commonly associated with six-shot revolvers. When playing with a single bullet, mathematically, the average number of consecutive pulls of the trigger before the gun discharges is 3.5. After a single spin, the probability of it firing is 1⁄6, followed by 1⁄5 on the second pull, 1⁄4 on the 3rd pull and so on, until if it failed to fire 5 times, the probability is 1⁄1 (=1) on the final pull. If the cylinder is re-spun after each trigger pull, the probability of firing remains 1 in 6 on each occasion, and the probability of it having fired after 6 pulls is about 66.5%.
https://en.m.wikipedia.org/wiki/Russian_roulette