I like the way the carpenter158 alloy fails. Even with 2 bolt lugs broke off the gun it still runs. I think Nickel boron or nitrided should be avoided since their failure mode seems to be to break in half. Just good old GI style bolt is the way to go. They last, they're cheap, their failure mode is non catastrophic and gives you lots of warning before the gun blows up in your face.
Generally, any kind of bolt failure on the AR is due to a bad interface between the barrel extension and bolt lugs, a proper fitting assembly shouldn't fail! That said, bolts should also be made from very high strength alloys, most that fail because of improper alloys are too brittle!
I don't think coatings or surface treatments in and of themselves contribute to structural failures. It's more the materials, machining, treatments, testing, and overall manufacturing process. I normally get mine from Toolcraft since they make replacements for .mil and source to a number of OEM manufacturers including FN for their portion of the M4 contract. I prefer nitrided since it cleans much easier than the standard coating. Also, it's a salt bath surface treatment and doesn't wear off. Honestly, not a real factor these days with cleaners like Carbon Killer, but make field maintenance much easier.
Never had a bolt failure, and always just run what is stock to the gun. Colt and Aero Precision here and never any issues. I never had a lot of use for fancy, just want it to go bang when I tell it to.
If you don't beat the shat out of it maybe it will last longer? (Turn down the gas) Bootleg Four Position Adjustable Carrier | Bootleg Inc
