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.
"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...."
It's compressed but it's compressed
in a specific direction. The blow-by has momentum. Air has mass and everything that has mass, when in motion, also has momentum. And Newton's first law says that everything in motion starts out trying to move in a straight line. Which in this case is >>>> toward the muzzle. The reason the propellant gas isn't backing up against the bullet is that it's in the nature of all moving objects to continue moving in the same direction. It was moving toward the muzzle when it escaped past the bullet and it will continue moving in that same direction unless and until something changes its mind.
By the time the bullet is fully obturated, there's in the vicinity of 10,000 PSI chamber pressure behind it. The volume of the water in an 18" 5.45mm barrel is less than three-quarters of a cubic inch, less than half an ounce. That little timy bit of water is just too minuscule to change the intentions of a gas driven by 10,000 PSI.
Before the bullet ever strikes the lands, there's already a rush of several hundred to a few thousand PSI of hot propellant gas entering the throat of the barrel (the gas always tries to outrun the bullet, otherwise it wouldn't be much of a propellant). If there's already water there, and since water is (virtually) incompressible, something has to give. Air and water can't both occupy the same space at the same time. So the highly pressurized air drives out the water, and there's only one direction for it to move in: toward the muzzle.
At which point the water also has momentum >>>> toward the muzzle. All the propellant gas needs to do from that point forward is keep the water accelerating in that direction and the bullet will remain essentially dry until it's clear of the bubble created by the muzzle blast. And there's no arguing but that the first thing to reach the muzzle in the video linked in the OP is
not the bullet but gas.
You also can see strong evidence of my claim of gas having momentum when it exits the muzzle. At first a little of the gas escapes through the ports on the sides of the muzzle brake but that's before any of it has got as far as the muzzle. The overwhelming majority of gas continues in a straight line. Even after it leaves the confines of the barrel, the gas still mostly travels in a straight line despite the considerable resistance exerted by the water it's colliding with. If it were being overwhelmed by the resistance from the water, it should have "bloomed" to form a more spherical shape. But even outside the confines of the barrel, it's still pretty pretty linear, sort of carrot shaped. So momentum is still the primary influence on its direction of travel, even once it's left the barrel.
If it's no different from a plugged-up barrel then the video obviously must have been faked because the rifle should have blown up.