Rob,
The JSB Heavies are surprising - my '30S shoots them really well too !
My HW30S has a larger than desirable breech
It is most accurate with the below (for me) :
H&N FTT (8.65 gr.)
Boxed Crosman Premier (7.9 gr.)
JSB RS (7.3 gr.)
JSB Match Blue (8.2 gr.)
JSB Heavies (10.3 gr.)
Have fun
Best regards
Russ