So in conclusion:
BB has a ideal FPS. The higher FPS the heavier BB you should use. Therefore, high FPS doesn't determine better range and consistency.
So if I did find the idea weight vs FPS with a 650mm inner barrel, the difference would so small it's not even worth it compare to a 510mm inner barrel. When I started airsoft my buddy told me 'Just go the longest inner barrel and highest FPS and you'll snipe like a real sniper!'. Definitely smacking him across the face.
Thanks for all the input. I'll use my bolt action to test out the topic mention here and hopefully come up with a reasonable report before I perform any purchase for my GBBR.