AR-15

The **AR-15** is the civilian semi-automatic descendant of Eugene Stoner's AR-15 design, the rifle that became the M16 and M4. Modern civilian AR-15s are a sprawling ecosystem of parts, uppers, lowers, and calibers built around a common lower receiver interface. It is the single most popular rifle platform in the United States, with estimates of 20+ million in private civilian hands. The appeal is modularity — a single lower receiver can host dozens of uppers across calibers, barrel lengths, and configurations — combined with the world's largest aftermarket for any rifle. The baseline platform is a direct-gas-impingement 16" carbine in 5.56 NATO, and that's what most shooters should start with. Everything else — SBRs, piston systems, large-frame .308 uppers, pistol-caliber conversions — is a specialization off that baseline.