Upgraded Spring for Swiss Arms TAC1 in .22 ?

What spring would you use in your B19 Chinese airgun, there are many guns that use this platform - Crosman Optimus, Remington Summit, Storm, Quest, Gamo 200/400, Swiss Arms TAC1 not to mention all the B19's out there.

I would like to up its power to the mid 700's with CPHP pellets. My new TAC1 is shooting in the 640's and want another 75fps ish ?

The gun is very nice and I truly like everything about it (although it is a bit loud) except I want a little more speed.

wll
 
Since I posted this 5 days ago, her velocity has increased a little bit. I also today removed the bear trap and that made a huge difference in the sound ... that thing was a rattle trap. Will put a little grease on the spring to quiet her down a little more, I hope another 10 or so %.

She is getting about 645+fps with JSB Exact 15.9 gr pellets, I would like in the 675's with those pellets ,but we will see. CPHP's are shooting out at 650-660 fps now, maybe a little more break in is needed ? I can say she cocks very smoothly !

wll2506