I've just finished HPAing my 850 (13ci Guerrilla Myth bottle and reg and Best Fittings adaptor). After a bit of messing about and testing I'm a bit disappointed with the results. When running on C02 it would fire AA Field 5.52 at about 560/570 fps on a nice warm day. Since doing the conversion it's only pushing them out at a max of 520 fps.

'Not a problem' I thought 'I'll just pre-load the hammer spring a bit'. Nope - doing that only made it worse . The more I load the spring the worse the fps gets. Am I missing something really obvious here?

I reversed the poppet spring from the reg and removed the pin valve from the Best Fittings adaptor as it was making the gun very inconsistent. Doing so reduced the shot to shot variation by some considerable margin, but I still have this weird set-up whereby increasing the hammer pre load reduces fps output. I don't want to mess about too much with the internals because as it stands it can be put back on C02 very easily.

I just can't work out why pre-loading the hammer spring reduces velocity?

Cheers

but I've always adjusted mine in the regulator.

won't give you any more performance once you have "enough".... If you plot velocity vs. preload at constant pressure, you will find a plateau, and all that happens is that as you increase hammer strike past that is you waste progressively more and more air.... I have seen SLIGHT reductions in velocity with increases, but basically the velocity stays flat once you have enough hammer strike....

Bob

Thanks for that - I'll try and shim the reg instead

Sounds like your reg is set way low probably 850psi. Also there are a few porting mods to the barrel, valve and the adapter that should be done with an HPA conversion.  The Dominator hammerspring cut to desired length works best with a shortened spring guide seat. Flow modding the internals is the same for Co2, mine goes 730fps on hot 85*+ days with JSB 14.3s after flow mod.s and hammer spring.

Hi Mate, I have recently added the same parts to my .22 850 and have had the same experience with average fps at 550. Did you get to the bottom of the problem? Im not looking to exceed the limit but I could do with a few more fps.
I notice you said you have removed the pin from the BF adapter to get more shot consistency.
Excuse my ignorance as I am new to air gunning and very new to modding, but how do I remove the pin?