Hi SPECTRE;
I got Rocky Mt. Arms to apply their BearCoat to my Sig P220.
As you may know, it's an epoxy-based coating. It proved quite fragile (wore off high spots, points of contact, breechface)and added SIGNIFICANT thickness to the gun's surface, so much so that many markings were partially obliterated, and more importantly, THE GUN COULD NOT BE REASSEMBLED after field-stripping, it was so tight!
After sending the gun back to RMA, the problem with reassembly was improved but not resolved, this in spite of having followed RMA's instructions to fire >100 rounds through the gun before field stipping the first time after the coating was applied.
On rifles, especially the AR, this probably wouldn't be so much of a problem, but the finish is still fairly fragile.
BTW, I've also tried the "solid film lubricants", eg. Metacol III, which contrary to advertising hype, are VERY fragile, moreso than BearCoat, and scratch readily.
Finding a truly durable black firearms finish or coating is difficult. "Black chrome" is nowhere near as durable as standard industrial hard chrome, while Walter Birdsong's "Black-T" is also easily scratched. Maybe the new Boron Carbide coatings will turn out to be "the ticket". (See http://www.bodycote-na.com/ if interested) Best.