With a fire burning, hold a lit match at the top of the opening of the firebox. If you see the flame being pulled into the fireplace, you'fe fine. If the flame comes back torward the room, you have draft issues and will possibly damage you're guns.
Traditionally i've heard, guns were hung over the fireplace because it provided a warm, less rustable atmosphere to put them.
If you have vent free gas logs, DO NOT put your guns over them. They put out huge amounts of water vapor and will destroy the finish on them....J.R.