OK, last time we had one of these questions, and I posted the fact that, over time, hard chrome will flake off the assmbly, and imbed itself in the walls of the upper, some guy came on and all but called me an idiot for posting that.
When I responded by offering to send him the early GI plated gas carrier key in my parts box, with half the plating worn off, if he would tell me where he thought the plating went, all I got was silence.
The plated assemblies aren't all that much easier to clean either.After all, most of the fouling is deposited on the bolt tail, which is easy enough to clean, and the inside of the carrier recess for the bolt, and that's already plated.
I promise you, if there was any significant advantage to hard chroming the exteriors the Armed Forces would still have plated parts. As far as I'm concerned, it's just another way for some supplier to take a few extra bucks out of your pocket. I've had the chromed pieces, and sold them all off years ago. Spend the extra bucks on a couple of extra mags. You'll get far more value out of them.[:D]