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.

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).