The top or bottom ribs on a double gun can become loose.  The solder holding the ribs to the barrels can get hot from rapid shooting, especially using black powder, and the solder can fail.  Or, a manufacturing process may not have solidly bonded and vibration could cause the solder joint to fail.  In any case, the problem needs attended to.

A simple and well known test for loose ribs can be preformed with the barrels separated from the action and forend.  Let the barrels hang from your index finger by placing the hook (the part that pivots the barrels from the frame when they are opened) and then tapping the barrels with a flick of a finger nail or some other semi-hard implement.  Care should be taken not to drop the barrels and avoid the use of an implement that could scratch or otherwise damage the barrels.

If the ribs are loose, the sound made by the barrels will be a dull "thud".  If the ribs are tight and the barrels are in good condition, they will ring like a bell.  If you don't hear them ring, have a qualified gunsmith evaluate them to determine their true condition.

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