And I thought the idea was to direct more of the sound of a short barrel (with lots of left over powder burning when the bullet exits the end of the barrel) rifle down range vs. around the muzzle and back to the shooter.
I have a couple of them on AR15 pistols. One's a Noveske PIG and the other is a cheaper one called a Kineti-Tech. It is two piece. The first piece screws on the barrel, like a flash suppressor and has various holes/slots in it, more like a compensator than a flash suppressor though. The second piece then screws onto the first piece and has only the big hole up front, towards the target.
They seem to work fine. I've shot them at the range, under the firing line roof/cover and they seem no louder than a short barreled .357 magnum (I've not shot my 4" M29 S&W under a roof/range cover in over 30 years, so I don't remember about it).
Link to a you tube video/test of a KAK flash can. We put a KAK flash can on my youngest son's .300 BO AR15 pistol but haven't gotten around to shooting it yet, so I can't comment on the KAK vs. the others I've shot.
https://www.youtube.com/watch?v=MB3CHwax3bI