I've got two jars of Flat Dark Earth air dry. I'd repaint the barrel for you since I have the equipment if you pay shipping both ways. I'd do the oven coat as well if you buy the paint. I do have the Elite series Earth color but that won't match exactly. I'm in the Denver area.
5/8" threads on a .680 OD to support a suppressor?
I would have used .030 (or whatever was called for) in brass shim stock around the barrel instead of tape, might give that a try, awfully thin wall to turn a tapered bushing for it.