I saw that you said world war 1 was the last war swords were used for combat in.. that is not technically true, because there was a man by the name of: "Mad Jack Church Hill Jackson" (is what his team called him) he rushed into battle with a scottish sword, long bow, and bagpipe, during world...
I have never heard of anything anything like it. But I like the idea, and would definitely want to read it. As it sounds like it would be si-fi/fantasy which is my favorite genre. :)
So long as used properly within the context of your story, I think guns are are perfectly acceptable in a fantasy story. My favorite author uses dragons in a modern setting. And he also uses some some si-fi elements, such as portals to other worlds.