When the Second Amendment was written in the late 18th century, the main weapons of war consisted of muskets and flintlock pistols. There are two very worthwhile things to note about this period in weaponry: First, it would have been very difficult for any individual to walk into a crowded theater and commit a massacre with one of these weapons, mainly because by the time the shooter had managed to reload the weapon, everyone could have already run out of the theater, or even escaped at a leisurely stroll [...]and this:
But previous history has taught us that perhaps the most effective insurgency weapon is the Improvised Explosive Device. If we are serious about defending American liberty against our own military, it's clear that we need our patriotic bomb-makers to have the freedoms they need to defend our country. As long as our government is monitoring and regulating purchases of fertilizers and other nitrates that could be used to make the explosives we need for self-defense, it's quite clear we can't have the freedoms we deserve to defend ourselves against tyranny.