I've never used anything like that. I keep an eye on the muzzle for accumulation of objects but have never noticed enough of anything to generate a concern. Shotguns get the worst of it due to the large opening, rifles don't seem to collect anything.
Heavy snow or freezing rain is a concern but keeping the muzzle pointed down prevents accumulation in that case. When I need to set the gun down by standing it against a tree in those conditions I drape a bandana over it. I always take at least one of those along when I go out in the woods, they've got a multitude of uses.