pj150 is right about Flitz. It is a sort-of standard for cleaning nickel guns. I've never heard of it damaging the finish.
But I'll add my voice to the rest: if you are about to photograph the gun or need it extra spiffy for some occasion, then go right ahead. But as part of the routine cleaning it's just a waste of time, and even though I've never heard of Flitz harming a gun, it is an abrasive and, at least theoretically, any abrasive will eventually start to remove the finish.