While the Beretta ARX-100 qualifies as a Modern Sporting Rifle, it most certainly is not a member of the AR-15 family in any way, shape, or form. Yes, it shoots the same ammo, and uses AR-15 compatible magazines, but everything else is much, much different.
You can think of the ARX-100 rifle as the Lego set of MSR’s. Everything, and I mean everything, is reconfigurable, adjustable, removable, or reversible without the use of any tools. Well, in fairness, when changing the ejection pattern from one side to the other it helps to use a cartridge to press the internal pin. Clearly, this rifle was designed for agency and military use. The ability to buy one rifle and easily reconfigure it for different applications, right or left-handed users, different optics types, and varying shapes and sizes of operators is a big deal for groups that have to issue one kind of rifle to lots of folks.