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).

Indications v2, v3, v6 does not mean the level of advance, but only shape of gearbox skeleton which was used and selected by the manufacturer depending on the capabilities of the body of the replica. For example, a models of the M4/M16 family, MP5, have a v2 gearbox, while the AK family have gearbox v3.

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.

Battery voltage is responsible for the frequency of shots. It must be remembered that, the stronger spring is in its replica, the higher voltage must have the battery, because the motor that drives the whole mechanism needs more energy to stretch the spring.

Battery capacity determines the time, which it accumulated enough energy to work gearbox. The greater  capacity, then replica will shoot longer.

NiCd rechargeable battery is characterized by high efficiency current-carrying capacity, but there are disruptive effect on memory. Should be unloaded and loaded in the full cycle, ie, it is recommended to fully discharge, and after unloading must charge the battery immediately without leaving it for a long time in a discharged state.
NiMH batteries have a higher self-discharge over time and shorten the life - on the other hand, they earn about 30% more capacity.

Hop-Uup system is used for overclocking bb during the shot (putting it in rotation), through which, it extends its flight path. Acting laws of physics make that bb lift up, while the gravitational force take it down. Appropriate adjustment of Hop-Up, will compensate for these two factors, which result in long and straight flight for longer distance and be greater focus shot (to improve accuracy).