Having a gun inside a thin-walled spacecraft filled with oxygen sounds crazy,
Having a spacecraft filled with pure oxygen sounds and is crazy. The Apollo 1 fire (1967) showed just how crazy it is. Which is why they don't do it anymore. Neither ISS nor the Russian capsules have a pure oxygen atmosphere. In fact, the ISS atmosphere is ground-level pressure with 20% oxygen. Only the EVA suits have a low-pressure, high-oxygen breathable mix.