I have played enough in the last two days, the conclusion is, there are some setting where FX really suck and there are some where it really rocks.
I actually was wrong with 730-740fps number. It was 630-640. So drop was up to 170 FPS. The worst setting is to open valve on max and set power around 4.
Even with 10 minutes pause speed drops significantly. I have managed to settle the speed at 750FPS +/- 2 with 25grn JSB king pellets and it didn't change after 3 hours. I keep valve open to a second line now and power on max.
If anybody is interested in difference between JSB King Heavy and JSB King Heavy MKII
MKII fits much better into the FX impact magazine. It fits much better into the barrel.
There is also a difference in output speed and power: Standard JSB KH - 620FPS, MKII - 640FPS (+20 FPS more) on those best settings I picked for myself for a short range shooting.
ps. Squirrels have never been happier. No more bad injuries.
