Just for the record, handgrenades existed at the time of the Constitution.
Private citizens could own any weapon in existence, many of the trading posts and forts were equipped with cannon and private arms the equal of those in government service. Many wealthy private individuals maintained their own private armories, not regulated by the govt. in any way, in order to defend their lands and workers against attacks by bandits and Indians. As long as those private, unregulated, arms were only used within the law there was no concept why the government would care. Except perhaps to ask for help in case of emergency.
Private citizens owned cannon armed ships which had pistols and rifles and grenades (and cutlesses and boarding pikes, ad nauseum) for defense. None of those items were provided for or regulated by the Govt. at the time. Now, if you committed a crime with your ship o' death, they'd hang ya for piracy.
See, back then at least, the idea was to criminalize the action, not the object.
As far as "what should private citizens be allowed to own now"? The middle ground standard nowadays is usually expressed as any weapons useable by an individual soldier. So machineguns, grenade launchers and light cannon, handgrenades that sort of thing.
However, we don't live in a vacuum either. You don't need laws against private fighters, or tanks, or artillery pieces as long as you enforce property laws. For example, I'd love to own an Apache, but I'm not licensed to fly one, I don't have anyplace to park it, and I don't own sufficient acreage to shoot its weapon systems without violating other peoples rights to safety and noise abatement.
If I owned a huge chunk of Interior Alaska and could feed and fuel the thing and use its weapons systems without violating others property, or game laws, or environmental laws then bully for me.
See, people turn into tunnel-visioned idiots when the concept of privately owned DD's and MG's come up, for no reason. There are plenty of Constitutional laws, Fed. state and local, on the books that will restrict ownership strictly on practical grounds. And again, if someone wants to slaughter a bunch of people a private MG is a less-efficient way then any number of effectively unstoppable bomb techniques. So there really is no logical reason to restrict ownership. Murderers will do what they want.
Oh, as far as nukes go, don't restrict the ownership per se, simply make someone demonstrate they have a place to detonate one in absolute safety before selling it to them. That's a reasonable restriction based on property rights and the libertarian "non-aggression principle".
