What power is it set at?
I find AA Fields 5.52 good up to 900fps if you are using more power then Bismags are worth a try. I still always go back to Fields though.
I have being trying Bobs 18g Exterminators which group well at 870fps so they are also worth a go.
If you want a Sample let me now and I will send you some, a few more that Bob will do for so proper testing MAKE SURE you clean you barrel first before using these though.
HTH Andy