The two barrels I've had in mine were horrible inside, lots of machining marks throughout. I good polishing might help it out a bit. You may have had a misaligment issue with the transfer port that caused the velocity change. I've had a new choked .25 barrel blank sitting on the bench for a couple weeks now, but no time to install it...
I just have one question, why you forcing such a high speeds when we all know that's not good (in most cases) if you want to have accurate airgun?