Back in the '70s I was an armorer for a mid-size (800 officer) PD. We did notice some nickel flaking on a very few Colt and S&W DA revolvers, mostly on the cylinder faces. Our lead armorer called S&W, and I was listening in. The S&W manager told us not to use Hoppe's #9 on nickeled guns. He said some nickeling procedures involve first copper-plating the firearm, supposedly to allow the nickel to better adhere. If the nickel gets cracked even minutely (such as on the cylinder face) the Hoppes can intrude and begin to eat away the copper, allowing the nickel to flake or lift. My understanding at the time was that this applied to nickeled S&W revolvers, but I have since been chastised severely by RKIs who say Smith never used the copper base, so I likely misunderstood.
On the other hand, no less a luminary than Elmer Keith observed in his Gun Notes column in G&A magazine a half-century ago that Hoppe's #9 was the only solvent he found would not throw the first shot out of the group after cleaning. I tested the theory in a half-dozen sniper rifles and found it to be true, though that is hardly a scientific or formal test.
