Just a flash hider. The real one works well to reduce recoil.
And they turn the minimal muzzle blast of an AR into something like that of a 7.62. On a 10.5 inch gun, indoors, it's ridiculous.
And, to clarify, the device pictured in the link is a TTO. Like the FSC556 with the 4 prongs milled off. The normal DNTC has two sets of ports, where the TTO only has one.
I'm a huge fan of PWS' CQB Comp, and saw an airsoft version the other day at Gearbox.
