The problem isn't the way that most people use AR15's (Which isn't in for their designed purpose). It's the way that a few people use AR15's... which is in the exact manner they were designed for. The way most people use AR15's could be replaced with any number of previous "un-modern" rifles. You don't need a gas fed semi-auto rifle to hunt. You don't need one to protect your home. The biggest difference between any "modern sporting rifle" and a traditional non-semi auto weapon is speed of rechambering the weapon and reloading it. That's what makes the biggest difference in mass shootings. The rate of fire between rifles between the Garand-era and the M14 / M16 era increased from 40-50 rounds per minute from a skilled marksman to 700-750 rounds per minute with the M14 and closer to 800 rounds per minute with the Colt AR15.
It makes it a lot tougher to kill multiple moving targets when you're having to pump / lever / manually cycle a bolt. And it's a lot harder to kill multiple moving targets when you are forced to reload bullet by bullet (like with a shotgun, a revolver, lever action or even a clunky clip-fed action). It's possible, but it takes a ton of discipline and practice, and it usually means having to carry more than one gun which is cumbersome.
There is literally no more reason for a gas-cycled, magazine fed, semi-auto rifle to be on the market available to the masses (usually without a permit) than there is for any fully auto stationary machine gun. You don't need an AR variant to defend your person anymore than you need a .50 Cal machine gun, a grenade launcher, or an RPG. Again, the only reason for those weapons is to kill as many people as possible in as short an amount of time as possible. It's not a "modern sporting rifle" and more than a diesel flamethrower would be a "modern barbecuing device"