If you've spent any time in gun forums or at a recent SHOT Show, you've probably run into the term "Lev-AR." It's not a single trademarked product but a nickname shooters use for a growing category of firearms: lever-action lower receivers built to work with standard AR-15 uppers. Think of it as a mash-up of the old-school Western lever gun and the modern modular rifle platform — same manual-of-arms nostalgia, same AR ergonomics and parts compatibility you already know. The Basic Idea Behi …