Both Advance Armament and SureFire were in the mix, but we ran across this one from Ranier Arms that included a video demonstration. The interesting thing is the geometry isn't much different from one manufacturer to the other -- with the open prong design dominating the look. But one thing is radically different about the Ranier Arms product: it's price.
At less than $50 per piece, it's a lot more approachable for ye olde civilian AR owner (or local SWAT team?) than one with a government credit card.