A "Barney Mag" is a reference to the 1960's Andy Griffith show. Andy was a
sheriff in a small town. His deputy was Barney Fife (Don Knotts). Barney was only allowed to have one cartridge. And, he had to keep it in his pocket. Barney had a bad habit of accidental discharges.
So a "Barney Mag" holds one round that you keep in your pocket until you load your gun prior to starting a stage.
Unless you have grey hair OR you watch AMC you really wouldn't know Barney Fife
OOPs! I didn't see RickB's post. Sorry.
I am the Christian Conservative that CNN warned you about!!