Soviet TKB-059 prototype 3-barreled rifle

There were a bunch of projects during the Cold War seeking to improve hit probability (as analysis had shown that most rounds fired don't actually hit) - generally, the idea was to fire several projectiles with each shot, to increase the statistical chance of at least one hitting.

The TKB-059 is probably the simplest and most basic attempt, by just literally tripling the gun and firing 3 bullets at once, but this obviously had the disadvantage of a lot of extra bulkiness, weight and recoil - it was just an experiment, more of a proof-of-concept (or attempt at one, anyway) that wasn't really expected to go very far. The West also attempted something simple, but from a different angle - the ammunition itself, with duplex & triplex rounds, where you'd have several bullets stacked in the cartridge itself, but those didn't work out due to accuracy issues caused by the several bullets interfering with one another.

Most other attempts were based on firing several projectiles in very quick succession - for example, the Russian AN-94 (which is after the Cold War, but it's still building on this idea) had a special hyperburst mechanism that would fire at a rate of 1800 rounds-per-minute (while most regular rifles tend to be in the 600-800 RPM range)