How these weapons differ

Semiautomatic and fully automatic weapons are distinct.

Semiautomatic: Semiautomatic rifles like the AR-15 patterned rifle, owned by millions of Americans, is the civilian variant of the military's M-16 and M-4 rifles. One squeeze of the trigger means one round leaves the barrel, which limits the amount of fire to how quickly the operator can pull the trigger. Semiautomatic weapons can legally simulate automatic fire with "bump" modifications, according to the Trace, a site that publishes gun-related news.

Automatic: An automatic weapon fires continuously as long as pressure is applied to the trigger and ammunition is available. Its mechanical system — used to feed, eject and ready the next round — is what makes it a machine gun.

What's the law: A provision of the Firearm Owners Protection Act of 1986 banned civilian purchases of machine guns, but it also grandfathered in weapons registered before May 19, 1986. ATF said about 175,000 transferable machine guns were in the national registry as of December 2015.

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