I tried tape. It helped but not as much to make a difference.
Is your rattle that bad? Unless you are a ninja the bolt catch rattle isn't significant to "give away" your location. Although it may be distracting, by the time a bad guy will hear it you'd be close enough to see the color of his eyes.
I've actually "patroled" w/my XCR in a place where even your footsteps doesn't give much noise signature, but even there your footsteps are just as loud as the bolt catch not to mention other equipment and enviromental noises you produce.
It is a mental thing. Like I said I've gotten to ignore it now. Tactically it is not a real concern, more annoying than anything else.
Powder coating doesn't soud like a bad idea. I thought about the liquid tape, even spoke to Alex about it and it's a risk to take if that thing jams b/c of that. But the powder coating probably will do alot to help, but I don't think it eliminate it completely.
Something tells me RA will eventually come out with a rubberized one. If not them someone else will once 20,000 or so XCR's have been sold.