because copper is harder than lead...
Copper or copper-plated? Neither would harm a steel barrel.
Copper-plated should perform the same as lead pellets of the same shape and weight provided the plating isn't too thick. The plating is supposed to prevent lead fouling. Also shouldn't produce lead dust; so your swimmers should be safe.
Copper pellets (if they exist) probably won't do so good. The head won't deform very easy increasing the required engraving force. The skirt would also not deform enough to the seal the bore behind it. Reduced mv and inconsistant. The barrel should be fine though.