A 400fps BB leaving an airsoft gun without any backspin will hit the ground pretty quickly (guestimate 45-55ft).
A 300fps BB leaving an airsoft gun with backspin can travel 2-3x further.
(I'm sure I'm going to mess up the physics of this and some university kid will wade in and lay a text book of equations on this...but this is what makes sense to me)
- the back spin creates lift and counter acts gravity from pulling the bb down.
- the "horizontal" push the bb gets from the spring/gas propells it forward
- the longer the "hang time" of the BB...the further it'll go...the faster it goes the less time it'll take to reach your target (if you've played a game you'll have noticed that it's not all that hard to "duck" a shot from a guy shooting at you from across the field)
- when the forward speed of the BB drops off to a given point...if there's too much backspin the BB will litterally climb like a fighter jet and take off "straight up"...if there's just the right amount the BB's flight path is straight out with a sharp drop off at the end when the speed has bled off.
The amount of hopup that you apply to a 400fps gun is not always the same as a 300fps gun. The amount of hopup you apply to a 0.20g BB is not the same as what you'd apply for a 0.30g BB.
So what gives it range? A fine tuned balance of backspin application, tweaked to suit the FPS of your rifle.
Factors such as the softness/"stickiness" of the rubber, surface of the BB and a whole plethora of other factors determine how consistently hopup is applied...but those factors ultimately affect your accuracy/grouping...not necessarily the absolute range of your rifle.
Hope that confuses things nicely!