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
Type of fire
Magazine capacity [pcs]
Hop-Up 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).
Happy with this gun for the price. Its big but light, and because its plastic it squeaks a bit but that's normal I think.
It has a working safety and a slide lock which looks like it should work, It has a cocking hammer which clicks back in when you press the trigger but doesn't function apart from that.
Slight manufacturing detail at the top of the hand grip, is slightly sharp edge under the hammer, but it looks like it would be an easy fix.
All in all Its worth the money for 250fps