I am thinking that it has to be the info that is in the program for your bow. I've done the same thing with a friend's bow as I did my own and the program holds true. However, I've never tried an arrow that heavy. I've ordered a chronograph and will run a few different arrow weights through it this weekend and see what happens.

I setup my program for the 365 gr arrow at 297 fps (IBO of 309 fps)and get 265 fps for the 515.7 gr arrow.

Maybe you should contact Larry and see what he says about this. I had some issues that he helped straighten out.