The use of energy weapons isn't actually necessary. Cartridge firing weapons will work, but there's a couple caveats. The first is that the ammunition has to be loaded in a vaccuum. Otherwise residual air that gets trapped can pop the bullets from the casings. A guy who ejected at 40k+ feet in an F105 mentioned this happening with the .38 he had on him. Other not so well known experiments showed the same.

Temperature changes are a problem too. On the moon for example, it's 200F one minunte and -100F the next. That's if one actually wants to use their gun in exo-atmospheric conditions.

Gravity, or the lack there of, isn't as much of a problem.

Moving to a rail gun system saves some of the ammo problems, but there's still issues with power, what you make it out of, etc.

The old failed Gyrojet was a solid idea and would have worked as a true space gun.

Reality has already exceeded most of the old science fiction, but that advances came with targeting systems and optics, and to a lesser extent with implementing precision guided munitions in a shoulder fired weapon ala the new grenade launchers.