As far as durability,I'd question most of the spray cover materials. Most will flake off after a short while.Especially on a hunting rifle being drug through bushes and whatnot. I have refinished FAl builds with several of the mentioned spray-ons and they all fell victim to bake out or flaking. They will look new and good for a while but most oil based paints will get soft as the oil in the paint heats.Much like the thinning effect of engine oil. I tryed engine block paint once and it held out pretty good.For a while.Once you heat the bbl a little,oils will excrede from the steel itself,also effecting the coating.Oil rubbing your gun will kill the Kote too. A better method is parkerising and then gunkote. The alumahide is tough but very thick. Can change the look of a gun from nice to cheesy real quick.
The pricing you will get on nice gunkote systems are scarey,but some are down to earth. Several online gunsmiths will do it fast and friendly.
Having done many park/gunkote jobs,I can honestly say that it only takes a few minutes to prep and bead blast a rifle.The parts,a couple minutes more. Then a hour or 2 to parkerize it.
Since it's an AR bbl,I would go with a spray on and only go to the HGs. That way,when it starts chipping and peeling,you can remove it easily and redo. You will smell any of the above mentioned coatings cooking out as you fire or target practice with your rifle. Good luck.......
jerryboy