one out of 5 bbs, goes down
 

I have v2 systema gearbox with mosfet and everything on a mp5 jg.
One out of 5 bbs, goes down after a few meters, the rest 4 go pretty ok.
What could be the cause?

So is it, like, EVERY one out of five (or really close to that) drops, or more of "about 1 out of every 5 i shoot drop)? if it's almost exactly like the fifth one out of every 5 shot or something to that effect it's probably gearbox. if it's more of a random thing then it might be your hopup or gearbox? i'm kindof stumped here, you might wanna open it up and take a look. mechbox.com is a VERY good guide to this.

Sounds like it might be double feeding every now and then. What kind of magazines are you using?

If it was double feeding, shouldn't 2 bbs come out at a time? It's only one but it just goes down after a few meters.
It's not always one out of five, it's more like random I think(3-4 going well,1 down, 6 well - 1 down, etc).

Possibly.

I have a similiar issue with my G36. Every now and then when I put in a magazine, it will feed one BB at a time but at very low velocity.. not sure why. Always fixed by reseating the mag.