Hydra Upper Assembly Features The Hatsan Hydra is an incredibly versatile air rifle system that gives you multiple caliber options in one single air rifle platform. With nothing more than the turn of a screw at the back of the receiver you can remove and replace the caliber of the Hydra. Just drop in the new assembly screw it into place and you re off to the races! Hydra Upper Assembly Specifications Brand: Hatsan Model: Hydra Type: Replacement Receiver and Barrel Assembly Caliber: .25 Fits: Hatsan Hydra Finish: Black