hi i bought a G&P m4a1 from wolf armouries, full metal, the gun itself is good and the only problem i have is that the rifle when on full auto fires for about 4 seconds then blank fires, regardless of it being a full mag ect i have to stop and wind the gun again, its not the mag as i tried 4 different mags, 2 of which were different makes to what i own, i think its the hop up, but could it be the battery? im not entirely certain it wud be sinces its only a 8.4 mini type. any help would be greatly appreciated