Looks like lead to me. Two really good options for getting the lead out of a barrel or chambers. The Lewis Lead Remover. Those things work great! If you cannot find a Lewis Lear Remover kit or simply don't want to pay for one, get some copper Chore Boy pot scrubbers. Make sure they are 100% copper and not cheap, copper plated steel. Cut some strands of the copper, wrap them around a bronze bore brush and proceed to strip the lead from the bore and/or chambers with no damage to the revolver or its finish.