Ok, I have officially debunked this rigged test. First off he lowered the H&K into the water with the bolt forward and a round chambered thus sealing off the the gas port and the chambered cartridge sealed that end of the barrel thus not allowing as much water to enter. Notice how little the amount of bubbles come up when he lowers the H&K. It's like when you put your finger over the top or a drinking straw and push it into a drink. The straw(barrel) doesn't take on any water.
Then when he lowers the standard AR into the water he has the bolt in the hold open featuring as its going into the water allowing the entire action,receiver,barrel,gas tube,buffer,ect to be totally flooded. No wonder he wore Ballistic helmet and shield. Notice allot more air bubbles cumming up and he then chambers a round under water.
On the shooting under water test i kept pausing it and you can notice the flash hider is literally poking out of the water the last second right as he fires. Also doing so causes the weapon not to cycle and he must recharge the weapon to continue firing above water. I seriously doubt the effectiveness of a .223 after passing threw only 1" off water,much less a barrel full.
Just do as they recommend and pull the bolt back a little to allow the barrel to drain when cumming out of the water, or just leave the bolt locked back so it drains instantly when you come out and you can instantly hit the bolt release being ready to fire faster than recharging the weapon like he had to.
The only thing I'll give the H&K is it can fire underwater, with the flash suppressor sticking out,because its piston driven and not gas impinged. For all we know the standard AR could do that if he didn't try to blow it up on the first shot.