update:
Both 33.6gr and 38gr NSA .254 slugs, which actually measure .255 at the base, shoot amazing out of the GK1
They have an R4 BC ~.088 with the first shot at 860ish fps and the 17th shot at 815ish fps when starting with a 350bar fill.
The accuracy is very very good.
I tried AVS .254 slugs, which come in measuring at a true .254, but they do not shoot well at all. I ordered some .2553 slugs to test. The GK1 seems to really like .255 ammo.
If you are looking to tune your .25 GK1 for maximum performance out of a single magazine with a full 350bar fill, 32gr to 36gr is a very good sweet spot for your ammo.
38gr or higher shoot well, but the velocity will be too low imo, with first shot at 825 fps or lower.
I will post some shot groups in the coming days.