Hatsan jet 1 .25 cal. power mods to get a consistent 650+ first shot velocity with 26gr pellets (Buckchrony used for test). Came to me shooting around 525fps with 26gr pellets. This is basically a new gun, only test rounds through it.  I have 3 jets and I am ready to let one go.  This jet 1 comes with a new, never been filled jet 2 cylinder pack.  Custom hammer spring installed, that is 3/8 inch longer than stock spring.  I  will include original spring if you want to go back to it.  I have improved the porting on the jet1 plenum to flow more air.  I also polished the hammer and the barrel for more consistent and increased velocity.  It is possible to get more, but this tune still gives you 12 shots on a fill/ jet1.  The Jet 2 cylinder should get around 20 shots or more per fill(not tested).  I get around 700 fps with a jet2 cylinder and this tune with my jet2. The jet2 plenum flows almost double the air by design.  I get over 700fps with my personal jet1, it is tweaked to the max and only gets 4 hot shots on the single cylinder.  I personally guarantee no leaks at time of sale, and that it functions as advertised.  The best thing about Hatsan pcps, is how easy they are to modify. Jet1 cylinder is hot swappable. You can swap 1 & 2 cylinder units with air in them,4 screws hold them in place.  Very fun little guns.