JSB manufacture alot of pellets for the trade. Known clones from JSB are AA field,Daystate, RWS Superfield to name a few (there are others)
You need to find the pellet and head size that suits your barrel - that is the trick. Clean the barrel between each pellet batch and then fire say 50 or so thru it befroe testing.
Is your bench rest indoors or out, as some of the match quality flat heads will be excellent indoors (H&N Match Finale, RWS R10 Match for example).
Fianlly, I do suggest you weigh and size each pellet, as small variations will not help in your discipline
HTH
Phil
AAS200T.177 & S510SL .177. FWB P70(diopter), P70 FT(Big Nikko) Steyr LP10, LG110(Big Nikko), LG20(diopter) . The gun is deadly accurate - shame about the user