This is a Slide Fire Solutions SSAR-15 bump stock. Donald Trump and assorted congresscritters have been threatening to ban these nationwide by executive fiat for months now. Several individual states have already made owning one a "felony." What is all the hype about?
Since the Gun Control Act of 1934, it has been illegal to own a machine gun without a special tax stamp and other red tape, making it effectively impossible for most people to legally own a fully-automatic rifle due to the tax, paperwork, and increased cost of such now-scarce items in the civilian market. Later acts in 1968 and 1986 added further restrictions. However, firearm enthusiasts still often wish to have the option to turn money into noise at the range, and entrepreneurs have developed solutions to get around the laws.
A machine gun is currently defined in law as firing multiple rounds with a single trigger press. Bump fire stocks operate by allowing the shooter rapidly press the trigger and fire many individual shots in quick succession. There are no springs or other mechanical aids, so the Bureau of Alcohol, Tobacco, Firearms, and Explosives (ATF, BATF, BATFE, Bad Attitude Toward Freedom, etc.) has reluctantly determined that this is an unregulated firearm part and not a machine gun by their definitions.
Donald Trump wants to change that. Various other politicians do, too. We will see.
How It Works
The small "interface block" replaces the standard pistol grip on an AR-type rifle. There is a hole for the safety detent spring, which needs to be removed from the existing grip. The stock itself has a locking switch so it can function like a standard fixed stock, or be unlocked to reciprocate along the buffer tube.
In order to use the rapid fire technique for which this stock was designed, the locking switch is rotated parallel to the axis of the buffer tube so the pin disengages from the detent notches. Then, the shooter's right hand pulls the stock against his (or her) shoulder while the left hand pushes forward on the forend. With the trigger finger in position, the forward movement of the receiver fires the rifle. The recoil back into the stock releases the trigger, but as long as forward pressure is maintained, the action will be repeatedly pulled against the shooter's trigger finger until that pressure is released.
Here's a Youtube video from someone who shows how it all works better than I probably described it:
Why People Are Scared
There is a common perception that banning stuff someone might find scary creates safety, that guns cause crime, that actual assault rifles are used to commit assault, that owners of semi-auto-only AR-15 rifles or similar firearms are in fact owners of assault rifles, and that anyone who owns one is clearly planning to shoot up another school or concert.
Why It's Nonsense
The AR-15 pattern the most popular rifle in the USA. However, rifles are rarely used in any crimes, and the AR is superb for hunting, pest control, target shooting, and competitive shooting sports. Regardless of its external appearance, it remains a semi-automatic rifle only, meaning it only fires a single round each time the trigger is pressed.
People have been bump-firing semi-auto rifles since long before these stocks were made available. The stock actually makes this practice safer, because the rifle is much better controlled, as it can be held properly and aimed instead of fired from the hip or by other awkward means people have been using for decades.
Starting well before the Clinton gun ban came and went, continuing while bump stocks sold like hotcakes, and coinciding with an overall trend toward relaxation of carry restrictions, violent crime in the USA has plummeted by about half. The "gun deaths" statistics cited by people who want to regulate them away dishonestly include suicides and self-defense as if they were violent crimes. The US does not have an unusually high suicide rate overall, and violent crime remains high in areas where the War on Drugs has created lucrative, and thus violent, black markets. Overall, the US has relatively low crime by global standards.
Sending police to enforce arbitrary bans on inanimate objects isn't fighting gun violence, it is endorsing gun violence. Most laws are just foolish opinions imposed at gunpoint, and that is not a civilized solution to anything. Bump stocks were used in a single high-profile crime, but that no more proves their evil nature than the use of a Toyota Camry with a body kit as a getaway car makes the Camry an inherently criminal vehicle, or that body kits turn them into race cars.
I don't plan to use a bump stock myself. It seems like a poor solution to a problem I don't need to solve. I want a rifle for single accurate shots. I think a bump stock is a silly range toy, and not a serious firearm upgrade.
That said, a ban on something I don't want to use is still a trespass against the rights of others. If there is no victim, there is no crime. If there is no crime, enforcing a law is criminal. Remember, slavery was once "the law," and it was "illegal" to be a runaway slave or help runaway slaves. Legality does not define morality.
So if you want a bump stock, you have every right to buy one. You also have every right to buy an actual assault rifle, machine gun, or other fully-automatic firearm. The laws only inform you the level of gun violence the government may inflict upon you for violating their arbitrary dictates. That isn't something to be respected as a legitimate authority, but it may influence your choices as a free individual in an unfree world.
I apologize for any typos resulting from composing this post on my phone. I will try to edit them as I find them.