Kilibru:
You can do the air pressure test and if you are satisfied that the ribs are solid and you wish to stop the solder balls or bead piece from rattling:
Take a syringe and inject linseed oil between the ribs, slightly diluted is best.........then stand in a corner on the muzzle for 24 hours..........then stand on the chamber end for 24 hours.......your loose solder balls or piece of bead should stick to the dried linseed and your barrels should be rattle free....
Clean the void prior to the linseed oil injection with acetone and let dry.
You can buy commercial syringes at Home Depot which will fit your rib hole quite nicely.
It will also protect the void against rust.......this is commonly done on welded aircraft chrome moly fuselages for rust protection and left in the tubing for the life of the aircraft, the holes on aircraft frames are welded shut after adding the linseed. They have used this method on aircraft since about 1915.
Best,
