Rifles redesigned to be manufactured as pistols are nothing new, but they are certainly increasing in popularity rapidly. The SBR is unlawful to possess in a few states—and where they are possible to legally obtain, they require hefty a tax stamp and registration fee. Manufacturers across the board have responded with a variety of models, with the AR platform clearly being one of the most popular.
Getting this platform to work with a short barrel while using a rifle stock has been problematic enough. Making one completely reliable with no stock has been somewhat elusive. Thankfully a couple of things have changed that equation in the last few years.
Probably the biggest change is the operating system. Depending on the design, the piston system lends itself to reliable functioning in shorter barreled actions. They run cleaner, often cooler, and the bolt speed can be controlled more easily. Secondly, the manufacture of ammunition has improved by leaps and bounds. Ammunition is cleaner, more consistent, and adds to the overall reliability of a pistol in this configuration.