Well..... Not really a thousand

. A whole bunch of them though over the years. In a nutshell, the pistol mag in a handgun was designed for a pistol, thus it could be reduced. The 30 round AR mags (or 20, 25, 40 etc.) were designed for a rifle and thus could only be reduced to 5, since that was the law for a semi-auto center fire rifle. To get a 10 round one, it had to be completely redesigned to be distinctly different for the AR PISTOL. They first tried to just pin a 30 rounder to 10 and mark it, but it didn't fly because it was a rifle mag adapted to a pistol, and had to be 5 rounds. So the redesign with a significant enough different shape was enough to pass mustard and be declared a unique pistol mag and could be 10 rounds. They are allowed to be used in any AR because the law states that it is based on what the magazine was designed for (AR Pistol so 10 rounds) and not what it is used in (AR rifle or AR pistol). Clear as mud now?