Would try some other pellets...JSB Exact Jumbos (15.9grain) work very well in everything I've tried them in.

That said, you have to keep in mind that the pellet is jumping from the rotary magazine into the barrel (not sure if the 38T has a forcing cone, but think it would)...any minor variation chamber to chamber will have an effect on this entry...any inconsistent "disturbance" of the pellet head will result in an increase in group size. Greatest accuracy will likely be derived by utilizing only one chamber for all shots.

Just my .02

Al