The American Constitution has changed a lot and it has never been obeyed to the letter. Nothing against the document which is an incredibly ambitious and positive undertaking, but the anti-terrorist laws from the last decade override the 5th amendment, the 1st amendment likewise.
They had to amend the declaration to include women and non-white people at a later date etc
The famous 10 Amendments are all pretty general, "the right to bear arms" doesn't specify what kind of arms etc, a lot of weaponry is excluded from the 2nd amendment, you aren't allowed to buy nerve gas grenades etc
Of course it has to be interpreted rather than just treated as gospel.
Big Ol' TL;DR here because I can get all history teacher geek in this thread and it is interesting to me.
Actually, those amendments to the constitution are a good example of just how hard it is to change the dang thing.
The amendments which outlawed slavery, gave black people citizenship, and gave blacks the vote were only ratified after the Civil War, and basically the Southern states were forced to ratify them as one of the conditions of being readmitted to the union. Collectively they're called the "Reconstruction Amendments" because they were part of the plan that was imposed upon the South, as the losing party, after the Civil War. In the case of slavery, it took a civil war to change this part of the constitution.
And women? The 19th amendment wasn't ratified until 1919, and women had been agitating for voting rights since before the Civil War.
Importantly too, those amendments involved making additions to the Constitution, giving certain people additional rights, not taking away rights that Americans already had.
The Bill of Rights is basically inviolable to most Americans (anti-terrorist laws aside, because I actually agree with you about that, but many don't, and, importantly, no one has actually attempted to amend the constitution with those laws) -- even some of your most hardcore liberals will take issue with the idea of repealing the 2nd Amendment altogether, and it just isn't, at the moment, a realistic solution to the gun problem. As Stil points out, guns are as much a cultural problem as they are a legal one. When you propose disarming a population and a good percentage of that population believes wholeheartedly that it is their god-given right to bear arms, they're not going to give up those arms easily, and the conflict could easily be bloodier than any school shooting.
So we are probably stuck with the 2nd amendment, but it is mostly up to the Supreme Court to interpret it, and the SC has been called upon to interpret the 2nd amendment, and other amendments, many times. The constitution is NOT a document that has one strict interpretation and yes, there the SC has had to make exceptions -- the "fire in a crowded building" rule, various rulings about prayer in school, etc. The SC historically has tried to stick with what the founding fathers intended with their bill of rights, but since they are dead and we can't ask them, we are left trying to define what the spirit of the law is. With regards to the 2nd amendment, the SC has tended to side against stricter gun controls in recent years (since the 70s anyhow), due basically to the power of the NRA, but that wasn't always the case.
It is possible that we could go back to a different interpretation of the 2nd amendment, one that focused on the rights of states to form militias. It is also possible that we could see bans on certain types of weapons, enforced background checks, laws about storage of weapons, or other constraints.
Ultimately though, I think in order for there to be a big challenge to the way the 2nd amendment is currently interpreted, there would have to be a massive shift in American culture. Right now there is a lot of fear in America and not a lot of faith in the government. People aren't going to give up their guns in a time of uncertainty. Maybe if the economy turns around and we have a new era of peace and prosperity, people will feel less need for their firearms, but I just don't see it happening any time soon.