Assault weapons are not made for hunting or for defending oneself; they are meant to inflict massive human casualties.
Define "assault weapon". Fully automatic weapons are already ostensibly banned in the U.S.--manufacturers cannot produce new ones and importers cannot bring them here from overseas. Those that are already in existence can only be purchased with special dispensation from the federal government in a process that costs an extra few hundred bucks and registration of the buyer's fingerprints. It is not permitted to give them to another person or will them to a relative after you die.
But none of the mass shootings have involved fully automatic weapons. They have all involved semi-automatic rifles, like the AR-15, which is literally no different from a modern hunting rifle except in two regards: it looks more "tactical", and it can be more easily modified due to wider availability of after market parts. It would still be illegal to modify it to fully automatic, and I'm not aware of a case where an illegally modified AR has been used in a mass shooting. But any hunting rifle can be modified in the same ways; it's just that the parts are not widely available to make modifications.
The AR-15 and other tactical rifles can inflict massive human casualties, but the overwhelming majority of people who own them do, in fact, use them only for target shooting, self defense, and to control pest animals on farms and ranches. It is important to note, however, that nearly any rifle on the market can inflict massive human casualties under certain conditions. Take away the manufacturers' ability to produce ARs, and you're still left with literally millions of them already in the hands of private owners, plus hundreds of millions of rounds of ammunition, and an only slightly more difficult ability to produce massive human casualties with other types of rifles and semi-automatic handguns.
These rifles get a bad rap because they are popular and they are black and "tactical" looking. The reason why AR-15s are more likely to be used in a mass shooting is not that their capabilities are that much different from other rifles, it is that they are the most popular rifle on the market, relatively cheap to purchase, and easy to change parts on. Demanding an end to the production of ARs in order to stop school shootings is like demanding an end to the production of the Toyota Corolla to stop car wrecks. Just because Corollas are more likely to be involved in wrecks does not mean they are less safe--they are just statistically some of the most common cars on the road. It's the same with ARs.