What Goh says. It's an "amendment" so clearly the constitution can be changed to move with the times. There's rule of law now, it's not the wild west, there's no threat of invasion and if there was it would not be repelled by locals with no military training and small arms and as for fighting off your own government a few handguns aren't going to do fuck all against tanks and drone strikes.
Fact is that gun bans have been proven to work in both the UK and Australia, countries that are broadly speaking culturally similar to the US (when compared to China, India, Japan etc). Not banning guns boils down to the fact that Americans like having guns. And Sandy Hook proved that they like having guns more than they like children being alive.