Come end of the day ... another reason so many of us try MANY pellets finding the one/s that chamber firm w/o deforming skirts while being seated, while providing good accuracy.
My Tx200 (mk3) seems to like 4.52s, both JSb exact and AA field diablos, both about 8.4 in weight i believe. It took me a while to get the knack of loading the pellets, but they were never too hard to get seated once the head of the pellet was in. I used 4.51 AA diablos, and they weren't much different accuracy wise but my gut feeling was that the 4.52s seemed a better fit in the barrel. So my personal 2 pence (2 cents?) would be both give it some time to get used to loading in the somewhat constricted space of the breech, and try some smaller pellets before you try altering the barrel.