Bump stocks are now illegal in some places. Ten states have passed bans or restrictions on the devices since the Las Vegas shooting…. But in most of the country, bump stocks are still legal and available for sale.

The Bureau of Alcohol, Tobacco, Firearms and Explosives, the ATF, had already looked at the question of whether existing federal law banning machine guns also applies to bump stocks. It decided it didn't. After Las Vegas, there were calls for legislation that would clearly ban the devices, but that idea didn't go far in the Republican-controlled Congress.

After the Parkland mass shooting though, President Trump appeared to weigh in against bump stocks.

The NRA called for "additional regulations." So the ATF once again started the process of reviewing whether bump stocks should fall under the existing machine gun ban.

During that review, the ATF received nearly 36,000 comments from the public, many from people who'd never heard of bump stocks before Las Vegas....

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Many people wrote in to the ATF to oppose the ban on principle. Michael Hammond, legislative counsel for Gun Owners of America, says he worries about what a bump stock ban could lead to.

"Our question is whether or not this is an effort to go the back door and ban semi-automatics by regulation," Hammond says.

Here's the thinking: a bump stock is very simple, mechanically. Some hobbyists have achieved the same effect with simple homemade devices — even a rubber band. And if that's all it takes, Hammond says defining bump stocks as illegal machine guns could mean that a future administration would interpret that to classify as a machine gun any semi-automatic with the potential of being bump-fired.

He thinks that could be the first step in an end-run around the political decision in Congress not to ban semi-automatics.

"If every time we don't like what the federal law does, but can't change it — we just throw the federal law in the trash can — you're going in a very dangerous direction," Hammond says.

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