Hi, maybe i missed somewere but which pellet you use?
I got two rifles one of them Walther LG300 wich can make groups at 50m around 1,5 cm with JSB express, FT position, but a small breeze can increase it dramatically, but with JSB heavy can be less the 2,5 cm even with a small breeze.
The same with my HW77k. With crossman premium light the accuracy is very good (around 2 cm at 50m), but in a small breeze increasing up to 5-8 cm.
With JSB Heavy it is much better groups i can have.

Sorry for metric units, and for my english