I’m surprised the extra back pressure didn’t cause it to blow up. It effectively increased the weight of the projectile by quite a bit.
Blow-by is running point for the bullet.
There's always some propellant gas in the barrel moving ahead of the bullet. All cartridges (except some specialized target shooting rounds) are loaded so the bullet has to move a short distance after it is dislodged from the cartridge casing before it meets the barrel's forcing cone and begins being driven onto the lands. But because there's already so much pressure inside the casing, just the few hundred PSI needed to break the bullet free, some propellant gas rushes past the bullet before it begins to "obturate" or seal the barrel.
Even after obturation the "seal" is not perfect and as pressure behind the bullet rises, propellant gasses continue to "blow by" or sneak past, particularly down in the corners of the grooves.
So once the firing sequence begins, a bubble of gas gets in front of the bullet and remains there until the bullet has exited the muzzle. So it's not the bullet pushing aside the water in the barrel, it's the blow-by doing it. The amount of pressure cooped up behind the bullet isn't much different from when it's fired in air.
Watch the video closely and you'll see gas issuing from the muzzle for several frames before the bullet emerges. Even that doesn't happen until it's outrun the muzzle blast, which in this case appears to extend the better part of a foot past the muzzle brake. So the bulett already is a foot or so downrange before it runs into water again.
RE: the action working underwater, one reason the US Navy SEALs opted for the HK416 rather than the M4 carbine is that the HK is an OpRod gun, same as the AK47/74, not direct impingement, as was Stoner’s original design for the AR platform (and the current M4). If a DI gun’s gas tube becomes filled with water, it can cause hydrostatic lock, which can cause a kB!
If you’ve seen the many MythBusters episodes on firing a gun underwater, they typically use an M-1 Garand, which is also an OpRod gun.
Seems to me that the blow-by would then be greatly compressed thus putting additional resistance on the bullet. The bullet may even go past the blow-by and touch the water. No different than a plugged up barrel.
I doubt a shotgun shooting 3” slugs would would fare as well. I’ve blown up a shotgun in my youth and they don’t have quite the margin of safety.