[Drakker]

FPS is a measure of speed: how fast a BB is going.
Joules are a measure of energy: how much energy the BB is carrying.

It takes more energy to move a heavier projectile, thus at the same FPS, an heavier BB carries more energy (and thus will hurt more on impact).

Heavier projectiles will lose energy slower than light projectiles. This mean that at long range, after a set amount of time, a heavy projectile still carry more energy than a light projectile, and will actually have more FPS than a lighter projectile. This is why heavy BBs will fly farther. But to do this, you need a hop up setup that will give enough backspin to your heavy BBs so that they stay in the airs for at least as long as the lighter BBs.

So, yeah, FPS is mostly irrelevant when you want range. You need a great hop-up setup and heavy rounds. For accuracy you also want the gun to be as consistent FPS/power-wise as possible, which is why MultipleParadox warned you to not mess with you very consistent setup.