Angstadt Arms has released an interesting integrally silenced (suppressed, if you prefer) barrel for 9mm AR carbines. It uses a full-length barrel with venting ports (similar to the MP5SD system), combined with a tubular shroud 1.5 inches in diameter covering the entire barrel. There are no baffles; the gas behind the bullet vents into the silencer tube, expands, and then once the bullet has left the barrel is all vents back through the ports into the bore and out the muzzle. The ports are intended to both allow the silencer to function, and also to reduce muzzle velocity of supersonic ammunition to subsonic levels.

In my testing (which matches with what Angstadt has published, the velocity reduction puts most generic 115gr 9mm loadings just barely into subsonic territory, while +P ammunition remains significantly supersonic. All 147 and 150grains ammunition, however, is subsonic by a nice margin.

The Vanquish is not the quietest silencer available, but it's far from the least quiet, and it's remarkably economical. Angstadt offers it as a plain barrel/silencer, a complete upper, and a complete firearm, in either 10.5 inch or 16 inch length. For someone looking for a pleasant-shooting 9mm carbine for recreation or competition, I think it's a very appealing option - less expensive than buying a silencer, barrel, and muzzle device independently and in the 16 inch configuration, a single tax stamp gun as well.

Disclosure: Angstadt send me this barrel for free, and I fitted it to a carbine I already had.