At the great risk of sounding lame I add this writeup.

A hop-up device is a mechanism that is added to airsoft guns which puts a top spin on the BB as it travels through and exits the barrel.

As the projectiles used in airsoft guns are already spherical, putting rifling grooves on the barrel would be useless, a hop-up is installed instead to make the BBs trajectory rise slightly over the bore line to increase the range of the gun's zero and hence also increase its effective range.

By tweaking this hop-up mechanism the gun can then be zeroed in at any range appropriate to the different BB weights and muzzle velocitiy combinations the user decides upon.