fx400 accuracy

Mine is unregulated and shooting @around 930fps i get 55 shots that are very useable with hardly any poi change.Do you know what speed your pellets are travelling at?

We might need a bit more info if anyone is going to be able to help, is it .177 or .22, did you buy it new or second hand, if its second hand perhaps the previous owner has adjusted the hammer spring tension? Is it the power adjust model or an earlier model?
A regulator will give you more consistent velocity across your shot string but finding the right pellet for your particular rifle will help with the accuracy, what pellets are you using, are they of a suitable headsize for your barrel? Do you have a chronograph or access to one as you really need to know the velocity. Yeah, I know a lot of questions but I was new to airguns a few years ago and had to go through all this also and still learning,lol.
I have a Royale 400.177 un-regulated and I get approximately 120 shots from 220bar down to 100bar on power level 3 with hardly any shift in POI at 25 metres using H&N Baracuda Match pellets and my Royale 500 with a regulator installed will do 65 shots ( JSB Kings ) at an average 930fps before it falls off the reg. at 140bar.

Edit; I see its a .22cal, you answered before I could.

Have you chronographed a shot string through your gun?
My 22 cal. FX-400 (unregulated, on high power) has a very noticeable bell curved shot string. To shoot within a 10 fps ES I shoot between 180 & 140 BAR which provides me ca. 60 shots.

Lik Kris said, with the bottle full, the hammer spring can only do so much to release the air sending the pellet down the barrel, as that pressure fails off, its able to strike the valve harder sending more air down the barrel, thus increase pellet speed. So when you get a Chrony you can figure out at what fill pressure the hammer spring is most effective (at its peak) then your shot string and pellet speed will be the most consistant.