The two assault rifles used in the San Bernardino shooting were purchased legally. But with small tweaks, they were altered in a way that made them illegal and more deadly.
It's a problem that faces California which has some of the strongest regulations on guns in the country, including laws on assault rifles.
"The gun makers found easy ways around the law," says UCLA law professor Adam Winkler, who adds that California may move to close those loopholes. "It could be that it will make efforts to get around the assault weapons ban harder."
The way it works is that the rifles themselves are not necessarily banned, but specific features they can be sold with.
For example, it's illegal to buy one with higher-capacity magazines or a mechanism to swap out magazines faster.
"You can easily produce a firearm that didn't have characteristics," says Winkler. "It does slow down the reloading process, but it doesn't make it impossible"
But it's easy to modify that mechanism, and high-capacity magazines can be purchased across state lines in states like Nevada.
Then that same rifle can shoot off more rounds in a shorter amount of time, making it more deadly if used in a mass shooting.
Despite that, Winkler says it's difficult to outlaw assault rifles altogether.
"It makes sense to ban firearms if they're unusually dangerous," he says, "but the problem with the assault weapons ban is that many of the rifles that were banned aren't really unusually dangerous. They're just like any other rifle."
He adds that restricting the amount of ammunition people can purchase is also problematic.
"Law abiding shooters use a lot of ammunition," he says.
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