I don't think I said it would ever stop a mass shooting. What my laws would do is keep stolen guns off the streets to some degree. It's great to rob a house and if you're unlucky you get a year or two. Is it that great if you take a gun or three and you're now looking at six or seven years. Not so great for your typical thief. Mass shootings? Nothing stops them, not even a complete ban and confiscation. You'll never get all the guns, never. With everyone helpless (unless you're the Karate Kid) the most simple of guns could cause a slaughter and people have been making zip guns for years. Maybe swords would make a comeback but then again a maniac with a sword can cause a lot of carnage.
Someone once asked the question on a survey. You're one of a hundred people on an island. You don't know anyone else around you. You have a choice to make. Choose, do you want everyone to have a gun or to just have one gun in the possession of a random person (yes, it comes with ammo and it is a revolver)? The vast majority understood and decided that they wanted everyone to have a gun which made everyone equal again.
How many Hollywood movies have we seen (you'd think the STARS would notice this) where a single man with a gun became a tyrant over the innocents.