JG MP5K what is this bit?

[CiderPunk]

So i took apart my new JG mp5k to replace the spring - it was firing 370, and have got it back together having replaced the spring and adjusted the AOE and everything, so now it fires... 50fps. not my finest work! 

anyway when i took it apart this sprung out and I've no idea where it goes or what it does:View attachment 43319

 

[Prisce]

@Samurai has one, he may be able to help better than I.

 

[Samurai]

I don't remember seeing one like that in it.

 

[CiderPunk]

oooh i was looking for one of these exploded diagrams for ages! very helpful thanks, I have the version with the fold out stock so it might be specific to that, It can't be anything critical,

cheers