IIRC the Aftermath stuff out of the box is around the 370-380FPS mark. I think they may be quoting with either .12's or taking the "highest" fps outputted instead of the average.
First thing you should do though is take it to a gundoc to get regreased and shimmed (reliability thing).
For BB's .25's or .28's.
ಠ_ಠLess QQ more Pew Pew
READY TO >> RACE