A lot of misinformation here.
The problem with some gun cleaning solvents is that they contain ammonia. Ammonia really likes copper. It turns your cleaning patch green (with envy?
) It shows you its working.
Roy, our only real source for the information other than looking ourselves says the factory didn't use a base layer of copper on nickel plated guns. Refinishing companies often do use a first coat of copper to help the nickel adhere to a steel frame gun. Ammonia and steel have no real affinity for each other. Likewise with nickel. You could store ammonia in a nickel can if you could find one, and you'd have no problem.
If you've been shooting copper jacketed bullets, a cleaning or fouling remover containing ammonia will remove it. You see that as bluish green color on your patch. Ammonia has no ability to soak thru the nickel if its an intact plating. The only problem you could have is if you've got a gun that has been refinished in nickel, the refinisher used copper before he plated it in nickel, and the copper now has cracks or copper exposed.
How WD40 got into this discussion is interesting. It contains no ammonia. Its also a product that has changed its formula over the years.
Originally, it was the 40th formula tested in a search to displace water. That was long ago. At that time it was a varnish based substance. When it dried out, the varnish turned to ...... wait for it.....varnish. So yes, it did form a gum. Over the last few decades, the varnish (which, by the way, did an excellent job of displacing water) was replaced with stoddard solvent. It doesn't gum. Of course if you've got a gum deposit that has turned to rock, the WD40 in its current iteration will probably soften it. But then so will CLP or any of a host of other powder solvents and cleaning agents. Under those circumstances, blaming WD40 when any other agent will do the same is foolish. And note that a good cleaning with it or anything else will actually remove all the gum - end of problem.
But what we're really talking about isn't a good cleaning. Its the half way jobs people do because they're too lazy or stupid to do it right. And this only on formerly abused guns. There is absolutely nothing wrong with giving a dripping wet gun a quick shot of WD40 to protect it on a very temporary basis.
WD40 doesn't disable ammo, or if it does, its one of the least offensive products in this capacity. Back in 1980 (just yesterday to some of us) the American Rifleman decided to do a pretty comprehensive test of WD. And all other gun cleaners and solvents they could find. The basically sealed in plastic bags a bunch of ammo, soaked, doused, even submerged in every gun cleaner and oil they could find. Then at regular intervals, they fired the ammo.
Some of the chemicals did a fair job of killing ammo. WD40 wasn't one of them. It didn't affect the test ammo much at all. If you don't believe me, go find the December 1980 index of AR. It will direct you to the correct month (I think maybe October, but I'm not certain.) Then read it and weep.
So in summary, you can use WD40 to clean your S&W nickel plated guns. Or your blued ones. And you can use Hoppes #9 to clean and remove copper fouling. If you want to be prudent you can wipe it off and then smear a pleasant amount of CLP on the gun. Or you can just clean the gun with CLP and be good to go. Remember, CLP is a specification the military uses. The brand names have come later. And CLP stands for Cleaner, Lubricant, Protectant. Really. Who'd a thunk it.
If you want to remove the ugly black burn stains (not black powder), try cotton cloth, some CLP, or any other powder solvent you can find and rub a bit. It won't dissolve it, but it will make it less offensive, if it really bothers you, you're in trouble. I've never found a way to remove the burns. I leave it there as long as its not building up.