Not to mention wrist and ear-shattering recoil.
Which is why the birds head grip, and most users recommending lighter loads. The angle of the grip diminishes recoil. Nobody is saying use 3" magnum loads, if anything there is already an adapter for mossberg to load 1 3/4" shells.
BTW, taking a store clerks opinion of what to load and not researching what Mossberg owners are discussing? OP should know better. That's like asking the mainstream media about Trumps visit with Putin.
Look to the history of these in use - witness protection. Close range heavy projectile loads to transfer significantly higher foot pounds of force than almost any reasonable carry pistol. There is lots of leeway in that, same as the XM177, you don't need huge magnum rounds to do that. 5.56 out of a 10.5" barrel is considered sufficient and that configuration was made GI back in 1965. Now we are once again catching up with an old concept firearm and the public is behind the application curve. It's not about magnums from a 14" barrel when you are less than 21 feet from a target.
Let me put it this way - the infamous Miami FBI fight? Would things have gone so badly if the agents had these?
No doubt there were some who disapproved of the Fitz specials back in the 1920's. Which are now revered as "the snubnose." When you carry on you and need firepower, you shorten the barrel but the ranges involved reduce the loss of velocity and you are still in the ball park of over 1,000 foot pounds of force.
If you are standing in your bathrobe in your front yard over your newspaper staring down your neighbor's pit bull, do you want a 12 ga slug or a .22? I can ask that knowing that there are more than a few incidents where 9mm, .40, .45, and even larger calibers failed to stop an attack.
There very much is a proper time and place for a short barreled shotgun for personal protection. Those in denial are really arguing cultural acceptance issues rather than applied ballistics, and would impede your right to keep and bear in order to create social tiers of superiority over what you can own.