You missed the point. There are shot guns out there that can hold more than 3 rounds. You ban ARs, people will buy the next available, firearm of choice. You might as well ban all firearms. And that is the gun control that eventually happens in the liberal mind set.
Okay, now that you have fired your shotgun empty, now reload. That is the difference. And the other reason ARs are used is the price. I can walk into any pawn shop and buy a good one for less than 400 bucks and it operates as well as the most expensive. And the rounds and mags are tiny in comparison to the same amount of Shotgun Rounds in a mag fed shotgun. Plus the mags on the AR are pennies in comparison to high capacity shotgun mags. You want to carry 120 shotgun rounds in mags around you you had better be pulling a little red wagon. The cost of what was shown in the vid was thousands, the cost prohibits it's use for mass killings. Let's face it, the only reason some idiot isn't shooting up schools and churches with a M-249 Belt Fed (SAW) is the price. Almost anyone can afford the gun but the registration and storage makes it expensive.
Meanwhile, the cost of the shotgun that has more than 3 to5 rounds is staggering. An AR-12 box fed
Shotgun made in the US will cost you in the excess of 1000 bucks. You might get a Turk made one for right around 500 bucks but you really don't want one of those. You can expect to find a few used US made AR-12s for around 500 to 1000. Another problem is, unless you buy all your accessories from the original manufacturer, they may or many not fit.
The weight of the AR-12 with at least 10 rounds is heavy. Not something you want to be trying to run, drop and shoot with. And the mags are long. I saw a picture of a 28 round version and it made almost a 90 degree circle and would break if you as much as sneezed at it.
The AR-12 is a monster to fire. You lay out 5 fast rounds out of that puppy, you are pretty well done in for awhile. And that is with an experienced shooter, not some pissed off fruitcake. Yah, Yah, I know, you are a friggin Rambo but most of the rest of us are not.
Trying to use a Shotgun to kill 50 or more people means that you are going to be spending quite a bit of the time reloading on MOST shotguns. While the AR-12 may sound like the one to use, it's really not. It's expensive. Meanwhile, the Model 870 costs a fraction of what the AR-12 does and you can easily find one for less than 150 bucks in reasonable shape but the reloading time is still going to be too time consuming.
You Rambos (in you minds) may dream of defending your home against a horde of Commies but that just ain't going to happen. If our Military loses that bad and there are hordes of armed Commies roaming the streets your single shotgun with it's pitiful 5 rounds only tells the horde where you are at and they will be right over. So you got off your 5 rounds fast. This is going to be the last action in your life as the AK is far better at this than your shotgun ever will be. And the AR is better than the AK (some disagree). Shooting up a church of a school is about the same as that horde. Each horde member is capable of killing or maiming a huge number of people fast, reload, and continue to kill. In a firefight, your shotgun will get out 3 to5 and then have to be reloaded. Record seeking Fruitcakes can't afford the 16 round shotguns so they use the under 1000 buck shotgun and get maybe two or three. Meanwhile, the fruitcake that used the AR can get in the upwards of 50 dead and over 400 injured.