EDIT: Scroll down to see countervailing data compared to what I talk about here first.
The disparity in homicide rates and gun control laws showed "gun legislation, on its own, means little in terms of gun violence," the Insight Crime analysis found.
I'm on the left on most issues, but in this case, my fellow leftists are wrong. If only saving thousands of lives from gun deaths were as simple as passing a law!
Overall, the most interesting thing about the gun issue, I think, is that automatic weapons and rifles are used to kill far fewer people than handguns.
One source: http://www.politifact.com/truth-o-meter/statements/2013/jan/18/facebook-posts/facebook-post-says-more-people-were-murdered-knive/
Another interesting fact is that mass shootings account for a very small percentage of overall gun deaths.
Source, which cites other sources: http://www.bloomberg.com/news/articles/2013-09-17/mass-shootings-fuel-fear-account-for-fraction-of-murders
So if we're trying to prevent gun deaths rather than headlines about gun deaths, banning assault rifles won't help as much as you'd think.
"But saving some lives is a lot better than saving no lives", you say, "so let's ban semi-automatic weapons! And if people will just use handguns instead, then let's ban both!"
But remember the beginning of this post: passing a law to ban a type of gun does little to affect how many people die this way, unfortunately.
It doesn't seem very common these days, but on this issue, the right is mostly right.
EDIT: I'd like to mention two more interesting facts -- about 70% of all US murders are performed with guns (source), and about 60% of gun deaths are suicides (source).
EDIT: Here's a very different take on the matter...
US v. Other Rich Countries
...Americans are 20 times as likely to be killed by a gun than is someone from another developed country.
Source, which cites good data: https://www.washingtonpost.com/news/worldviews/wp/2012/12/14/chart-the-u-s-has-far-more-gun-related-killings-than-any-other-developed-country/
Hard to reconcile all this data! Comparing the US to other rich countries does seem more reasonable than comparing us to Honduras and other countries that don't make the US look like an outlier at all, as Wikipedia's data on this issue indeed does, where the US is 13th in gun homicides... after 12 poor, high-crime countries.
So perhaps the analysis mentioned up above -- as well as similar ones I've read about before -- include rich, poor, high-crime, and low-crime countries all mixed together in their data set, and find little correlation between a country's gun deaths and that country's laws.
I can't seem to find good data that shows the correlation between gun laws and gun homicides, either among all countries or just rich ones, which is unfortunate, because that's the best data I can think of that would help settle this complex issue.
2015.11.03: Study: mass shootings are "contagious" and spread kind of like a disease.