Exactly. "I saw someone with a gun," is just fine. If the police ask you to describe it, don't just say "Oh, it was an AK-47," unless you know what one of those is. How about you, I don't know, use your words? "It was really long, it had a stand and a scope." That tells the police a lot more, and is more accurate. Hell, I've shot an AK-47 and I still wouldn't be completely sure.