I mean hardware in the RYO sense, SRs, triglides, barrel locks, etc. I sometimes add shrink tube to some of it, like this*:
Similarly, I sometimes sew nylon pockets/tubes around other hardware, or things like the UV keychain light I always keep on my rig to make cateyes glow, etc. All to reduce glare, and especially noise if it's in a place where it can hit other hard objects.
But have I missed something, or is there a reason no one has made rubber-coated hardware? I vaguely recall a few back in the steel era (maybe triglides on slings?) but don't seem to have any examples. Seems a good idea, so why not, when there is a large enough military market they make special mil specific colors for example?
*If wondering, and you don't have to, I make my own leashes for NODs, use 3/4" SRs to attach. Mostly because: old. No good store-bought ones available to me when I wanted my first one.