The proposed ban on bump stocks not only applies to a wide, vague range of firearm accessories, as Christian Britschgi noted this morning. It also criminalizes mere possession of those accessories, making owners subject to fines and up to five years in prison, even if they acquired the newly prohibited items before the ban was enacted.

In that respect the bill, introduced by Reps. Carlos Curbelo (R-Fla.) and Seth Moulton (D-Mass.), differs from, say, the expired federal ban on so-called assault weapons, which did "not apply to the possession or transfer of any semiautomatic assault weapon otherwise lawfully possessed under Federal law on the date of the enactment of this subsection." State "assault weapon" bans likewise allow continued possession of the targeted firearms, as long as owners register them with the government. Curbelo and Moulton's bill, by contrast, says "this section and the amendments made by this section shall apply with respect to conduct engaged in after the 90-day period that begins with the date of the enactment of this Act." That means continued possession after that point would be a federal felony.

The criminal penalties are especially problematic given the uncertain scope of the bill, which bans "any part or combination of parts that is designed and functions to increase the rate of fire of a semiautomatic rifle but does not convert the semiautomatic rifle into a machinegun." That definition pretty clearly covers the products made by companies such as Slide Fire Solutions and Bump Fire Systems, which harness recoil energy to help shooters fire without flexing their trigger fingers. But it might also reach other, less controversial products that can help people fire a gun more quickly, not to mention do-it-yourself solutions that are impossible to regulate but might make tinkerers vulnerable to prosecution.

"There are a hundred ways to make a semiautomatic firearm fire more quickly, and there is no way to prevent them," says Rep. Thomas Massie, the libertarian-leaning Kentucky Republican who leads the Congressional Second Amendment Caucus. "You can use common household items."

Even leaving aside the question of how the ban might affect the legal status of rubber bands, metal bars, or pieces of wood (depending on the owner's intent), Massie worries that the bill could prohibit products that may have nothing to do with the mass shooting in Las Vegas but are arguably covered by the ban. "There is an industry that takes your off-the-shelf firearm and improves the trigger so that you can be more accurate and shoot it faster," he says. "Those implements aren't designed to simulate full auto fire, but they could be read into the language of that bill."

For example, Massie says, "if it takes four pounds of pull to activate a stock trigger with your finger, these devices may reduce that to two pounds. Nobody has a problem with those, as far as I know, with the exception of all the liberals who want to ban all guns. People don't have a problem with those devices, but they could be read into that language."

Massie is also troubled by the retroactive criminalization of devices that people already own. "Are the manufacturers going to be compelled by the government to turn over lists of customers who legally acquired [products] that were declared by the regulatory authority to be legal?" he wonders. "This could set the precedent for a gun grab if you're retroactively banning these things."