I don't know why they even bothered - people tested the gun-in-a-vacuum routine as well as the gun underwater routine decades ago. Pretty sure I've read about both tests in one or another of the gun magazines I've been buying for 30 years. A cartridge is basically factory sealed and everything it needs to explode is contained inside the shell, whether the shell is in a vacuum, underwater or being deep fried.
got that wrong, btw, when Jayne declared he had to fire his gun from inside a spare, pressurized space suit.
Besides if you can't shoot a gun in space it ruins some really cool scenes in Cowboy Bebop!