So which should fit your requirements? The under rail launcher fro, HFC or from Show Guns? Both can be mounted to 20mm rails or can also be used as standalone backup launchers in CQB games. Toronto Airsoft has both of them in stock so check out this comparison video they made:

Fludblud sends a link to his review of the Show Guns Tactical Shotgun Shell Launcher which is available right now at some airsoft retailers... "Here is my comprehensive review of a rail mounted airsoft shotgun shell launcher by Show Guns Tactical, complete with live subject testing. The device is extremely simple to use, lightweight and perfectly sized to fit on the underbarrel rail mounts of various airsoft sidearms.

Airsoft Mike gets first dibs at the newly released Show Guns Tactical 20mm Grenade Launcher and here is his video review... "In this video, I get my hands on the all new Show Guns Airsoft Tactical 20mm Grenade Launcher! The Airsoft gas grenade mini launcher that fits on your Airsoft pistol and more!"