It is the spped of bb, while its leaving the innerbarrel of replica. This speed is measured in FPS (Feet Per Second).

1 FPS = 0,3 ^m/_s

1 J = 99 ^m/_s

1 J = 326,7 FPS

Replicas have spring, electric or gas propultion. Spring models need to reload after each shot (pull back the lock) to enter the ball into the chamber. At the same time it pulls spring back, to be able to give a shot. Electric replicas depends on the battery, whereby the engine drives the whole gearbox, and a gas models, push the BB by gas, which loads in magazine (in these replicas ther is not spring).

The length of the replca is measured between the two most distant points of each other. Different models have several of these values, because of its design allows to increase or decrease it length (eg.  folding stock or extended flash hider).

This is a value that includes the weight of the replica, magazine and other accessories that are include in set without a battery, charger, cleaning rod, etc., etc.