What's the difference between that and a regular Sharpie?

I used their black on to  take care of some scratches on my Stag, the stuff reminds me of the Rustolium that handles 1500 degrees.  Veru convenient applicator and I even used it on a touch up for my Rav 4.  the chit holds up buddy.

you might want to try the alum black instead for an AR.

I can tell you the difference between the Birchwood Casey aluminum black pen and a black Sharpie, the BC pen will dry much flatter than the Sharpie, I've used both to mark aluminum magazines and the Sharpie will stand out like a sore thumb when the light hits it right, the BC pen in black is a much closer match. It even does a pretty fair job of hiding scratches on the parkerized finish on my WASR receiver. Also, some solvent like Goo Gone will easily take Sharpie ink off anything, the Aluminum Black seems to be tougher. You can also get the Birchwood Casey stuff in a small bottle and paint it on.