You may want to consider what is going to happen if your reciever becomes pressurized with that plug in the FA hole. I suspect that it would be the weakest point and become a projectile that would blow back at your face. If you are going to skip installing a real FA then you may want to consider this http://www.weaponoutfitters.com/ar-15-m16-1/upper-receivers-parts/upper-receiver-parts/ar-gas-vent-for-suppressed-ar-rifles.html I have not used this product nor do i own it but I suspect it would be better than a half way attempt at plugging the hole.