He was claiming that you could not fire the gun in space because of the lack of oxygen, this is a valid way of thinking because obviously a fire can not happen without a supply of oxygen.
I said you could because modern ammunition has it's own oxidizer, modern ammunition is made out of charcoal, sulfur, and saltpeter. Because salt peter is rich in oxygen when the firing pin causes the chemical reaction to start it can keep going because the reaction itself releases oxygen. At least that's how I see it, even if that's not the reason why you can I know a bullet can still go off in a vacuum. It's the same reason we can fire bullets underwater, on YouTube there are also plenty of videos of guns being used in vacuum chambers!