We are allowed 10 round 223/556 pistol mags in AR pistols chambered in 223/556... You are also allowed to use that same mag in a AR rifle chambered in 223/556.
This above gun is legally registered as a pistol, and take its proper 10rd 225/556 pistol mags.
Picture below it is an AR rifle, which you can also use the above 10rd pistol mag in it, legally.
The whole beowolf argument thing is that
no one actually designed said mags, they simply used the existing 223/556 mags as such, hence the limit of 2rd for anything that handles .50 beowolf.
This is very different than loading 9mm ammo in a magazine designed for .40cal; which is ok (and legal).
Sure the law isn't clearly written, and you can still find "5rd" beowolf mags in the wild, but I wouln't risk it, you could lose alot in the end... just get legal 10rd pistol mags for your AR needs