I have a polygonal-rifled barrel on an AR-15 upper receiver.
Black Hole Weaponry makes them from raw bar stock in St. Helens, Oregon. The company just upgraded and updated their website, so it looks like most of the information on their polygonal rifling is missing. But... it's a three-groove 'clover leaf' pattern, like their logo - just not as pronounced.
They're reported to be fantastic barrels. But, mine has not yet been tested...