My take on this, Roberto, is that the barrel has nothing to do with it.
A smoothbore barrel can get better groups than that.
The gun's recoil on the other hand probably has a lot to do with it. As hytyger is suggesting, try de-tuning a bit.
If the velocity drops too much because it's a .25, you may consider buying a used, complete with breechblock, HW barrel off here, in .22. Not sure whether it would have to be a 90 barrel or if other HW break barrel guns barrels c/w breechblock will fit - you'll have to find that out for yourself.
Personally except in PCPs at 40-50 ft/lbs+, I am not a fan of the .25 - too much commitment needed!!
One other thing to check - the lockup of the barrel. *Possibly* the recoil may be causing the barrel to "open" which of course would be no good. However I would expect vertical groups in such case.
**WANTED**: WEBLEY PATRIOT MUZZLE END; Any Diana/Original mod.50 parts, especially OPEN SIGHTS