What phrasing makes the most sense in a story?

So I'm writing a story, and I usually put down dialogue how I'd say it in real life. In one part, a character says, "I'm guessing that's no normal gun," because people talk like that, even though it's not the way you should properly say it. My question is, is it too improper that it doesn't make sense? Should I write, "I'm guessing that's not a normal gun"? Or should I leave it the other way so it's more authentic? thanks!