@realcaseyrollins various reasons. Off the top of my head I can think of:
- Some types of weapons aren't legally recognised as "arms" but rather "destructive devices" and thus not protected by the amendment
- The amendment only guarantees ownership and possession rights, so the government is allowed to ban manufacture, import, sale, etc.
- Some restrictions aren't viewed by the courts as infringement in the same way that defamation/incitement/obscenity laws don't abridge your speech rights
These aren't very good reasons, but there are a few common excuses.
> Some types of weapons aren't legally recognised as "arms" but rather "destructive devices" and thus not protected by the amendment
Why not? What is the difference between a weapon, an "arm", and a destructive device?
> The amendment only guarantees ownership and possession rights, so the government is allowed to ban manufacture, import, sale, etc.
This makes no sense, as these regulations actively prevent ownership and possession. There isn't really a valid case to make that states that preventing people from buying guns doesn't prevent them from owning and possessing them.
- Some restrictions aren't viewed by the courts as infringement in the same way that defamation/incitement/obscenity laws don't abridge your speech rights
We know this, and this is the point of my post; what is the constitutional basis for this notion?
> What is the difference between a weapon, an "arm", and a destructive device?
This is defined in legislation, I think from the thirties, probably amended since.
> There isn't really a valid case to make that states that preventing people from buying guns doesn't prevent them from owning and possessing them.
Right, but AFAIK there isn't a legal precedent that such workarounds are covered by the amendment in the same way that e.g. poll taxes and literacy tests are covered by the enfranchisement amendments. It does have some effect though in that the government cannot make you give up a gun you currently own (contrast with Canada, where it's mandatory to surrender guns added to the prohibited list) or stop you from walking about with them.
> We know this, and this is the point of my post; what is the constitutional basis for this notion
I think generally the "constitutional basis" is taken to be the combination of preceding clause about the well-regulated militia, the general welfare clause, and the elastic clause; but that's more of an excuse and the underlying basis is really the acknowledgement that an absolutist interpretation isn't such a good idea any more. Compare Japan's constitutional prohibition on having a military: once they had two nuclear-armed communist neighbours, they began to interpret the rule so they could have this "it's not really a military, honest" self defence force.
@realcaseyrollins it's a hard problem to modify anything in the Bill of Rights. The founding fathers are held in such high esteem that people react almost as if you're proposing to repeal and replace a chapter of the Bible. I think we might get there at some point, especially now that there's a greater awareness of some of the less praiseworthy aspects of their lives, but it's not imminent.