There is provided a decoppering agent that is incorporated into the propellant to remove copper from the rifling of the internal bore of a gun barrel. The decoppering agent consists essentially of a lead-free pulverized additive mixed together with the other propellant compounds. We have found that a mixture of tin and bismuth is a very good decoppering agent. Especially in the ratio 42% Sn/58%Bi, the melting point for this combination is 138 degree C. Much lower than each of the components. It can be all kinds of suitable tin and bismuth compounds. The bismuth and tin either vaporizes or liquefies when the propellant is ignited and either embrittles or dissolves the copper deposits facilitating removal.