I wonder if the floorplate on the M21 is actually the same composition as the receiver. They have known bluing issues on that part.

On mine there is a small area where the blue flaked off. It didn't wear or abrade, it flaked off like a paint would that didn't have good surface preparation.

The receiver itself has no such issues, and I don't carry guns by the metal if at all possible. This is a skeet gun and has never been in the woods.

"The price of good shotgunnery is constant practice" - Fred Kimble