If you can’t find them you might try some mx24 paint. I’ve used it before and it worked pretty well. I’ll do that to my front sight if I can find another oem post that glows as bright as the rear.
I was told last year the tank wasn’t large enough to do the Benelli M4 barrels in Armorlube.
I’d probably just have the parts Cerakoted in carbon black. Avoid the threaded parts of the magazine tube and the interior. Avoid the chrome barrel extension, seat ring which seats against the face if the receiver, and gas piston channels.