+ 1 for heavier pellets. The heavier the better - as long as those are (true 8 times of 10) JSBīs.
Keep in mind that the heavier/longer the pellet the more oomph is needed to get it into proper rotation. Like someone mentioned the sweet spot for most airgun pellets (and for all JSBīs iīve ever tried) is at 275-285 mps. Not all Guns are capable of putting the heaviest pellets in that velocity though so use the heaviest pellet your gun is able to set to 290 mps and tune for the best velocity for your barrel/gun from there. Yours may like 280 mps instead and so on.

Itīs a shame that the barrel makers donīt do barrels with steeper rifling since that would help those guns unable to get the heavier pellets upto proper speed. The faster/steeper rifling would do it instead.