Belive me, I tried everything possible to make this rifle shooting better...but no way. For this reason I'm thinking to swap the barrel, it's a sort of last chance.
Probably part of the problem is the pellet choice; in this caliber the market is quite poor, only the Diana dome 19.75 give acceptable groups. Other brands/models are simply useless.
Anyway .25 barrels on springers/gas ram are particularly pellet-fussy, especiallly if you go up with the power (I own also a terrible W&S Patriot same caliber).
On pcp irguns things goes different, I shooted a Rapid 12/250 @ 50FP and it makes beautifull groups with 30 grains Baracuda (up to 100 yds or so...).