Its a common fault with the G&G M14 to have a bad airseal at 1 time or another on the nozzle/rubber joint.
I have suffered from it with all 4 of the ones I have owned, and even had it on a mates CA I was re-building for him.
The only cure is a new hop rubber, making sure the lip is the longest you can get, and then carefully re-build it.
The first time I suffered with this I thought it was me or something I was doing wrong but after about the 6th or 7th re-build trying to use the same rubber, and several second hand reclaimed rubbers that I knew worked fine in other guns, I finally got pissed off and bought a new 1, fitted it, problem solved :
I have suffered from it with all 4 of the ones I have owned, and even had it on a mates CA I was re-building for him.
The only cure is a new hop rubber, making sure the lip is the longest you can get, and then carefully re-build it.
The first time I suffered with this I thought it was me or something I was doing wrong but after about the 6th or 7th re-build trying to use the same rubber, and several second hand reclaimed rubbers that I knew worked fine in other guns, I finally got pissed off and bought a new 1, fitted it, problem solved :