Weight changes: Lots of experiments and reading says you can figure 1 fps for every gr weight change, up or down. When Bill T. shot the elephant w/ Emax w/ 900 gr arrow setup, he got about 250 fps. 900-350= 550 gr/5= 110 fps less than advertised 350 fps for stock bow, fff, and stock arrows [350 gr] I know the math here ain't quite right, but that's ALOT of gr difference.