I disagree. The best weapon for a mass killing by one crazed individual would be a semi-automatic rifle with lots and lots of ammunition, which is exactly what we tend to see in these incidents.
Either way, it is clearly not illegality that keeps weapons out of the hands of criminals and crazies. Nobody is calling for the repeal of the 1934 Firearms Act, whatever the intellectual arguments against it. That's because machine guns and explosives are not what the good or bad guys (or crazies) demand for the job at hand.
You aren't going to like this...I have an absolutely killer argument, so to speak, against this point of view. It is true that now, here, in the USA, crazies use semiautomatics of various length, which they CAN get, but not grenades or other explosives, which they cannot get, not nearly as easily as they can get the assault rifles and Glocks with big magazines.
However, what do the terrorists use basically all over the world day in and day out? They sure don't use assault rifles...........
They use explosives. Every single day, in Iraqi markets, on the Turkish border with Syria in refugee camps, in Egyptian crowds, thrown at Afghans queuing for jobs, or Afghans sitting in a circle meeting, in Africa, in Tunisia last week, in Israeli restaurants, and on and on. Years and years of explosions to kill groups of people. Because that is by far the best way to kill masses of people if you are going for a high kill rate, and they always are. They mostly make their own explosives, but probably would LOVE to have a few cases of USA manufactured grenades: talk about quality control!
I don't know why you say grenades wouldn't be useful to throw into classrooms, work areas, theaters, etc. if someone was crazy -- isn't that what they are FOR? I read that soldiers threw grenades into Iraqi stone houses all the time to clear them out. You throw them at groups of people, and they all blow up, limbs a-flying.
All over the world anyone who wants to blow up a lot of people chooses bombs: cell phone or pressure switch controlled often, but grenades would work, that's why soldiers carry them. So suppose people could buy them at Walmart? They would, right? Everyone else in the world uses bombs, and if our crazies could get grenades, why would they fool with assault rifles? Much too slow. At Virginia Tech the mass murderer Cho used guns, but he went from classroom to classroom like Lanza did. Think of the improved efficiency and kill rate if he had had a manbag full of grenades from Walmart! Throw one in and keep going, throw another in another room, keep going.
The rest of the world has a HUGE problem with mass murderers, and they pretty much all use explosives. So grenades would quickly become the weapon of choice if crazies and terrorists here could get them freely under the Second Amendment.
So there is a problem about the Second Amendment already being infringed, since logically, grenades are clearly protected, but somehow --- we don't get to buy them at Walmart anyway.