A bullet from any production weapon, of any caliber and muzzle velocity will lose most of its energy within 5-7 feet because of the properties of water, some of them illustrated in the video, as well as the resistance of water as we’re all well aware here. I looked at the slow motion of the action cycling, and this is indeed a 7.62x39mm "AK-47" and not a 5.45x39mm "AK-74." Sure, a 5.56mm round from an M-16 or a 5.45mm round from an AK-74 will tumble when it encounters water, ballistic gelatin, human tissue, etc, but so will 7.62x39.