That vid doesn't help anything, but it shows the round getting out of your gun. Side shot vid of the ejection port would have been better.
I just worked on a Stag 20" rifle gas barrel, over the last two weekends, for my gun pusher. The gas port was too small. It was a carbine stocked setup, and had recoil system problems, too, but that shouldn't be your case. If you want to eliminate any guesswork on the recoil spring, use an Armalite EA-1095 spring or a Sprinco Red spring. Then, you'll know for certain that the recoil spring isn't the issue.
Check your gas port diameter, and come back with what it measures. Use numbered drill bits, and use the shank of the bit to measure - not the drilling end.