Rody wrote:Gunpowder however is special because it already contains the oxygen needed for its combustion. There is no reason why a gun should need oxygen other than what it already has.
This much can be said to be true. As can be seen with any internet google search, gunpowder will burn underwater, in space, and in other non-standard environs. With this in mind, it is a simple matter to note that any standard gunpowder-based weapon would work perfectly fine in space.
Flamethrowers would not work in space, however, due to the fact that oxygen gas (O2) is necessary for a flame to work. Combustion is by definition a chemical reaction between a fuel and an oxidant. An oxidant is a chemical compound that readily transfers oxygen atoms. Thus, flames require oxygen.
Space is a vacuum, meaning that their are no gases in space. Thus, it is impossible for a vacuum to support a flamethrower.
However, should we delve into the realm of science fiction, you could conceivably blast oxygen in a pressurized stream in space, followed by a flamethrower resulting in a long sheet of flame. However, to do so would require extreme engineering which is not available in the current day and age.