Courtesy of one of my consulting clients, I recently had the opportunity to shoot more than 120 rounds through an AR-15 equipped with an SSAR-15 SlideFire Solutions stock. These are legal "bump fire" stocks that slide 1/2-inch, allowing you to very rapidly re-press the trigger, under recoil. The effect sounds just like full auto. It takes a few minutes to get accustomed to controlling the length of bursts. The trick is using a slight forward pressure on the foregrip.
As with full auto, don't expect much accuracy after the second shot in a burst. (I was able to keep two rounds inside a 6 inch circle at 10 yards.) The stock can also be locked in the rearward position, for traditional (non-bump firing) function. Each stock sold comes with a copy of the ATF approval letter, certifying that rifles in equipped with this stock are exempt from the National Firearms Act $200 tax requirements. (Technically, under U.S. law, they are not full auto, because you are firing the gun semi-auto with individual presses of the trigger.) Don't be surprised if this ruling is reversed in a few years, on a bureaucratic whim, or at the insistence of higher ups in the Executive Branch. If Senator Schumer saw one of these guns in action at a rifle range, I think that he'd lapse into apoplectic spasms. Given the attributes of these stocks, be prepared to answer to authorities at a moment's notice. I recommend that you keep a copy of the ATF approval letter with the stock at all times. I was told that my host keeps his letter inside a forward pistol grip (with a battery compartment) that is kept mounted on his AR-15. That is prudent.
At first, I considered these stocks a novelty and just an expensive toy. But then I realized that for retreat groups, it might make sense to add one of these stocks to an M4gery or AR-15 in your battery. Picture a situation where your group retreat is being approached by a large group of armed, hostile looters. Odds are that most of them won't have any combat experience. The sound of "full auto" fire in a display of force might encourage them to flee and go find a softer target, somewhere else. (Your mileage may vary!)
One word of warning: A rifle equipped with one of these stocks becomes an ammo eater. They are expensive to shoot, so don't buy one unless you can afford to lay in an extra 2,000 rounds of ammo and a Beta CMAG 100 round magazine.