An interesting question ... After some thought, I'd say that guns do indeed work in areas of high magic. As much as I hate to admit it, I think a gun would beat a spell in a duel unless the caster was able to forego the use of a spoken word.

It's highly unlikely that would come into play in this particular roleplay though, as our setting is pretty much exclusively focused on Hogwarts and Hogsmeade. Not a lot of muggles around.