ive recently had problems with pellets. i use exacts in my pcp's because they shoot them the best and expresses in my springer.
i noticed that i was missing some shots that i really should have got, so i did a bit of testing. expresses would group really well for 5 or 6 shots, then i'd get a flyer. i put them over the crono. they would be very consistent, then one would be 30 to 40 fps down.
ive put it down to a crap batch and tried air arms fields.
they also shot very well and seem to be more consistent, so ive ended up swapping. i can also feel a difference in the firing cycle of the gun with air arms and expresses. expresses feel better, but dont seem to be as accurate.