Can you shoot a firearmer in space?

Guns do not need oxygen to operate, so outer space vacuum isn't going to be an issue. The gunpowder is entirely self-contained, and is not reliant on the environment. The firearmer functions best in vacuum. The bullet won't need to compress or move the barrel air, and will fly endless distances in space. On the moon, if you shoot a powerful gun at the horizon (with a bullet speed of 1,600 m/s) and then wait a while, you may get hit in the back of the head.