Before you do that...slide the back blcok back and take a lok at teh hole where the cocking rod goes into teh gun.
The hole should be allen key shaped, if not look to the left of teh hole.
If there is a small brub screw (allen key) there, then you have teh newer style of velocity adjustment called a Rex Dialler.
To increase teh velocity you have to unscrew it (anti clockwise), its arse about face I know.