Fixes for Hatsan 125 .25, barrel hole is over sized?

I was getting my hatsan 125 tuned and the tuner called me and told me my barrel hole is over sized. The pellet fits snuggly in the breach but after that the pellet fall through til the end of the barrel where it gets tight again. Kind of like a choke. The tuner said the barrel is so over sized its actually slowing the pellet down. He has experience with hatsans and says if I buy a new barrel from them there is a big chance I'll get another bad barrel. What would be a good fix for this? Are there any airgun barrels for these that would fit it ? I need recommendations.
 
One experience I had from two years ago was three consecutive .22 barrels that were very aggressively rusted to the point of visible pitting in the center. I think maybe some bluing salts got somewhere they shouldn't have and were not flushed out. When I finally got a rust-free barrel, it was it was reverse hour glass as your .25 is. That was the end of my interest in Hatsan.