MadBull Airsoft gives us a sneak preview of their upcoming grenade launchers licensed from Spike Tactical. By the looks of the image below, they will be making the 12-inch and 9-inch versions of the rear loading Havoc Launchers. The Spike Tactical Havoc Launchers have the proprietary STZ push button which they say to be 3 times stronger than existing lever lock systems.

Definitely, this is designed for the MadBull 40mm BB Grenade Shells, perhaps you can use shells from other brands but you will need to check with MadBull.

(Above Photo teaser:  ST 12" Rear Loading Havoc Launcher from Spike Tactical).