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The Soviet Union placed a 23mm autocannon in orbit as part of Salyut 3, a manned space station launched in June 1974:

https://en.wikipedia.org/wiki/Salyut_3#On-board_gun

Hard to be certain due to Cold War secrecy, but the consensus seems to be that they conducted several test-firings before the station was finally deorbited.

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using a 23mm auto cannon on orbit is more or less useless. There’s no way to reasonably shoot something when your relative velocities could easily be strictly greater than the bullets. Even if something is coming straight towards you it’s very difficult to aim when you have to take into account orbital mechanics. You would need a huge amount of RCS fuel in order to counteract the impulse from the gun. And even if you did shoot the gun if you miss, you now have a bunch of bullets that share your orbit and will at some point intercept your flight path again, if you don’t miss, you still have shrapnel sharing your orbital path.

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