Bruce is correct about the lead away cloth. They are pretty abrasive. Using them once or twice should not cause a problem.
Most powder solvents are OK on factory nickeled guns, as there is no copper undercoat. Leaving them on the gun for long periods could etch the nickel plating, but using them to clean should be fine.
WD 40 works pretty well at dissolving carbon, and it won't hurt the finish to soak it in WD 40.