JUst reread yuour original post. One possibility is that there is some loose matter in the rib cavity,thats been there since the gun was made. Held in place by the resin fluxes that were used ,the hot water used in blacking has allowed it to be free and so is now what is commonly refered to as a"devil" because its a devil to get rid of.Usualy a bit of solder. If it was rust then you would get a sound like sand trickeling up and down as you tip the barrel.The guy who blacked it should be able to do some thing about it . Being involved in all aspects of barrel work myself I cannot say its any ones fault if this is the case,I'm afraid to say its just one of those annoying things that life throws at us.
