There is a big difference between a muzzle brake, a moderator and a (*^%.A muzzle brake is nothing more than an extension on the barrel that is not much more that a handle to assist cocking and make it purty. It has next to nothing to do with reducing the "report" or mechanical noise on any airgun and is not intended to.A moderator (sometimes called and air stripper) such as is on the Gamo Whisper does suppress the sound of the expulsion of the air coming out of the muzzle of the barrel by breaking it up but is not an actual (*^% and the reason that it passes the test is that it cannot be removed from the gun (without destroying it) and placed on a powder burner to suppress the noise. It does suppress the report of pellets that break the sound barrier such as the PBA's.A (*^% is just that... a device that silences the report and is constructed for just that purpose usually using a series of springs and baffles in a sequence in a tube that is specifically designed to silence the report.
I wouldnt recommand one. I bought a tko but no longer use it because its not worth the risk to me.