I have one that only works with carrying handles. It locks into the hole in the carrying handle with a detent ball. Made of plastic, doesn't look real expensive. I'd post a pic but I have no idea who makes it. It was provided to me at work.
For individual use..I think they are great.
However from a training point of view I think they are dangerous.
My reason being..I can't see the chamber of a students weapon with it on and muzzle control goes out the window with most when they try to remove it.
As an instructor..I have to many other things to be concerned with and don't need the added responsibility, worry of 6 or more muzzles pointing everywhere.