2 minor I think issues

[aftershock]

Firstly the gun is a cyma045 aks74u which I got in trade for my tm desert warrior and the ak itself has only seen 5skirmishes since new and 2with me.

First skirmish at badlands it chronos 270fps on a 8.4 stick second chronograph at ucap it dropped 100 fps to 160fps on a full charged 9.6 .. This is beyond me as it's never happened to me before with any gun.

Also what mag I have in determines weather I can use full auto or it it shoots semi only.

She's a new is gun and I believe it may be minor but any ideas on simple fixes for it?