Yes, I know there's no way.
It's a misnomer to say 'a 500 FPS rifle', more apt would be a 'x.xx joule rifle' because the energy, measured in joules, is what yields the velocity of a projectile (energy x weight = velocity or something) depending on it's weight.
A rifle shooting 400 FPS with .2s will shoot 365 FPS or so with .25s.
See what I'm saying here?