Lower

The lower receiver is the serialised part of an AR-15 and therefore the firearm in legal terms — the upper, barrel and everything else are unregulated components. That is what makes building a rifle possible: you transfer one part and buy the rest freely.

Stripped lowers need a lower parts kit and a stock assembly. Complete lowers arrive built and are the sensible choice if you do not want to fit a trigger and chase detent springs across a bench. Either way it transfers through an FFL, and how you eventually configure it decides whether you have built a rifle or a pistol — a distinction that matters more than the parts cost.