Historically, selective enforcement is recognized as a sign of tyranny, and an abuse of power, because it violates rule of law, allowing men to apply justice only when they choose. Aside from this being inherently unjust, it almost inevitably must lead to favoritism and extortion, with those empowered to choose being able to help their friends, take bribes, and threaten those from they desire favors.
Of course, if you’re not David Gregory, and don’t have David Gregory’s connections, the D.C. attorney general is not nearly so understanding.
Tens–likely hundreds–of millions of such magazines are lawfully possessed in the United States by law-abiding gun owners. More than 1 million PMAGs are backordered from Magpul. Brownells sold through three-and-a-half years worth of magazines in three days.
Magazines like the one Gregory displayed on Meet the Press, and Magpul’s PMAG, are not complicated; they are made up of a box and a spring. Given the sheer number of standard capacity magazines in the United States, and the infrequency of mass shootings (they are not becoming more common, and your chances of being killed in a mass shooting are about the same as your chances of being struck by lightning), these magazines, as well as the AR-15 rifle (and all semi-automatic weapons) are in common use for lawful purposes.
If in common use is to mean anything at all, it must cover the AR-15–America’s most popular rifle–and standard capacity magazines.