It would be even funnier if it was an old model of a gun, those that had no oxydizer agent and therefore required external oxygen to be ignited, of which there is none in space. Imagine the astronaut feeling dumb after pulling the trigger and the bullet just slowly, awkwardly floats towards the other astronaut's head after being pushed by a metal pin with no ignition.

"Wait, your gun's an old model?"

"…always has been, unfortunately."

And in a few thousand years he'll collide with some unlucky science vessel