The latest announcement from ZShot are the licensed Knight's Armament QDC/CQB Barrel Extensions. You can choose between the long and short versions and these are sets, meaning that appart from the Barrel Extensions, they come with Flash Hiders for you to use with so you can use the quick detach feature. Pricing for both is at US$99.95.

Knight's Armament QDC Barrel Extension

Knight's Armament QDC/CQB Barrel Extension