Thats a little weird Sam - you are seeing a disproportionate drop in velocity between those two pellet weights compared to other rifles. With the heavier pellet performing better.My 125 gives me 740 with the 25.4 pellets and 840 with the 20's.Energy is pretty consistent between them.Yours is 50 fps less with the 25's, but 120 fps less with the 20's.....27 fpe for the 25's, dropping to 23 fps for the 20's.Like I said, that just seems odd.
Finally got 200 rounds through my 125 .25 cal. Dieseling has stopped and it seems to have settled in. Set up the crony today and was getting a consistant 730-720fps with hatsans cortex 24.70gr pellets. Had a few odd balls as there fit isnt the best. As of now i am not going to tear the gun down, but will keep shooting. One thing i find odd is there web site lists this rifle at 950 alloy and 850 fps lead. But the box said 750fps. It is the springer version. So far im happy