I've run into those hard endplates too. And it was odd considering they were Colt's no less. They weren't glass hard like nitride finished end plates, but they were hard enough that when I attempted my usual staking job on them the section I was staking started to cave down between the castle nut and the receiver. Of course after that you have to go through the trouble to change out the end plate do the whole process over again with a new one... It can get pretty frustrating.
Knock on wood, I haven't run into an overly hard phosphate finished gas key yet and I hope not to.
Anyways, some end plate staking that I've done with the hammer and punch method (3 to 5 light taps), I try not to go overboard and blow through the edge like I see so many do.
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