What liner are you using?
Funny enough I see lots of people shooting 18gr amazingly well from the pellet liner in their MK2, but mine won't shoot them well no matter what the velocity/reg etc.
25gr are also a pointless endeavor out of my impacts barrel. 16gr are another story and they're exceptional around 930fps.
The slug liner in my Crown shoots 25gr very, very well at 860fps and 18 well around 890-920fps. Haven't been able to get a slug liner for the impact yet and both guns are shooting well enough I'd prefer not to risk anything switching and swapping bits.