The AR-15 lower receiver is the foundation of your entire build — and the only part legally considered the “firearm” by the ATF. Whether you’re building a dedicated AR9 9mm pistol, a versatile 5.56 carbine, or a custom competition rifle, choosing the right AR-15 lower matters more than most shooters realize. With AR9 9mm and AR10 searches trending strong, many builders start with a quality AR-15 lower and then decide which upper to pair it with. At Zaviar Firearms, we off …