Granted, I see your point between the destructive nature between a HE grenade and a .380ACP. I also see your point on the likelyhood of harm of said munitions are greatly different.
That is all I am saying.
IME most full auto machine guns are much less effective in causing the type of catastrophic loss of life we see in school shootings. There are a few exceptions but most of the exceptions are highly situational. In truth well aimed shots with a rifle are nearly always more effective than well aimed shots with a machine gun (with a few exceptions).