As with most other rights, the Second Amendment is not an unlimited right. In his dicta from the Heller case, Justice Scalia noted that regulation or prohibition of "dangerous our unusual" weapons would not run afoul of the Second Amendment. What was left ambiguous, however, was what exactly constitutes a "dangerous or unusual" weapon. As is usually the case with restriction of rights, the best test when speaking about the regulation or banning of weapons is to compare the proposed regulation's benefit to public interest against its abridgment of personal liberty. A regulation that greatly increases public safety while only minimally affecting liberty is a reasonable one while the regulation that serves little or not benefit to public safety but greatly diminishes personal liberty is an unreasonable one.
Now, as to high explosives and weapons of mass destruction, I think it's fairly obvious that the benefit to public safety created by banning such things greatly outweighs the abridgment of personal liberty that such a ban represents. These weapons are so indiscriminately destructive that there are very few, if any, ways to utilize them that does not represent an obvious danger to innocent bystanders. I cannot think of any way that a person could use a brick of C4, canister of VX nerve gas, or suitcase nuke to defend himself/herself that would not pose a likely, if not certain, threat to the safety of anyone else around.
Firearms, however, are a different kettle of fish. Firearms do not generate indiscriminate destruction and, in skilled hands, a firearm can be used to destroy only that which the user wishes to destroy. SCOTUS obviously agrees with this notion as they have twice ruled that a total ban of private firearm ownership is unconstitutional.
Where the true debate lies is whether or not specific types of firearms represent a grave enough threat to public safety to justify heavy regulation or outright prohibition of their ownership. Currently, this debate seems to be focused on so-called "assault weapons" but, when the facts are considered, this should be fairly obvious because most of the features which have come do define a firearm as an "assault weapon" are cosmetic and have no bearing on the gun's destructiveness.
Another facet of this debate which has been discussed at length on this very forum is whether or not NFA weapons such as silencers/suppressors, guns with bore diameter over 50 caliber, short barrel rifles/shotguns, and most contentiously fully automatic weapons are destructive enough to warrant a ban. Many argue that fully automatic weapons in particular are indiscriminate by nature and thus warrant heavy regulations or complete bans on private ownership.
I personally do not agree with this sentiment. While fully-automatic fire, by its nature, is indeed more indiscriminate that semi-automatic fire, it is not so to the degree that explosives or WMD's are. A bullet fired by a full-auto weapon is no more destructive than one fired by a semi-auto weapon and the bullets will still only go where the muzzle is pointed. Furthermore, many fully-automatic weapons have a selector switch which allows the user to choose between semi-automatic and fully automatic fire as is appropriate for the situation at hand. Police seem to have found merit with select-fire weapons and I fail to understand how they should be any less concerned with indiscriminate fire than anyone else. Finally, just because a fully automatic weapon can cause indiscriminate destruction, that does not mean that it can only be used to do so. If I want a machine gun to simply burn up lots of ammo at the range because I think it's fun, I'm not hurting anyone else by doing so and thus I do not feel that it should be anyone's business but my own.
