I don't own a barrel that long, but it should work well. I run a can on a 10.5 inch barrel, and it's still loud to the shooter. Because of the bullet noise, it will never be ninja quiet anyway.
With ARs, excess gas vents out of the bolt carrier, and when I shoot my can on a 16'' barrel, my right ear still rings because of it. If you want to maximize quietness, you could install an adjustable gas block, and dial it down so it uses the minimum gas possible.
To somebody standing nearby, even as close as 5 feet, the guns are amazingly quiet.