How Bump Stocks Turn Semiautomatics Into Automatics, And Why It’s Legal

As police and journalists attempt to unravel the motives behind the mass murder in Las Vegas, one item that keeps being mentioned is a “bump stock.” It’s a tool that’s supposed to turn a semi-automatic rifle into a fully automatic one, without actually modifying the gun. But how it does this, and why it’s legal, isn’t entirely clear.

A semiautomatic weapon is one that fires a bullet each time you pull the trigger and automatically reloads the weapon for the next trigger pull. But you have to pull the trigger for each bullet you want to fire, hence the “semi.” A fully automatic weapon would just let you hold down the trigger and fire until you’re out of bullets.

In a bump stock, there’s a gap in the stationary stock allowing the gun’s recoil to drive it backward. There’s also a “support step” where the shooter can rest their finger. Then they simply pull the gun forward. When it fires, the semiautomatic action engages, reloads, and the trigger hits the shooter’s finger, firing again. The gun is “bumping” against the stock, and thus is automatic.

As to why bump stocks are legal, under the law, you cannot convert a semiautomatic rifle to a fully automatic one, yes, but that’s not, technically, what a bump stock is doing. Swap out the stock, a relatively simple matter for an experienced shooter, and the gun is a semiautomatic. An automatic weapon is defined under the law as one with an internal automatic mechanism. Since the stock isn’t an internal mechanism, it’s legal.

Whether that remains the case is an open question. The Federal Assault Weapons Ban, which lapsed in 2004, didn’t ban any class of weapons outright but instead banned certain features on weapons, such as folding stocks and bayonet mounts. There’s no provision in the Second Amendment that entitles anyone to a rifle scope or a bump stock, and Congress is free to ban or control those as it sees fit. It’s not clear if weapons with bump stocks were used in the mass murders in Las Vegas, but it really comes down to a simple question, with modifications such as this. Do we abide by the letter of the law? Or the spirit?

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