Like I said in a different post - one of my rifles is a Hornet Maxxim 22 with the IGT. It took me awhile to figure out which pellets it preferred, and some of the groups (@ 25 yrds) were terrible when I first started on pellet experimentation.
Like alot of people, I tried the 14.3 grain, domed, Crosman Premiers right off the bat. And although they grouped on occasion, most of my 5 shot groups were 1.5 to 2 inches, which is completely unacceptable at 25 yards.
On a whim, I decided to try a lead free pellet (GTO) at only 11.75 grains, and the rifle instantly tightened up to 1/2 and 1" groups, consistently. I've been experimenting with the RWS Hobby Match pellets @ 11.9 grains here lately. And although they group just as good as the GTO's at 25, they loosen up a fair amount at 50. But they are match type, so it's too be expected I suppose. However, the RWS pellets are MUCH cheaper than the GTO's.
Just my experience with the IGT system and Gamo rifle.