There's no denying the appeal of a compact AR-15. Shorter barrels make rifles easier to maneuver in tight spaces, more comfortable to transport, and increasingly popular among home defense enthusiasts, competitive shooters, and military veterans who appreciate the familiar feel of a shorter platform. But in the United States, shrinking an AR-15's barrel below a specific threshold doesn't just change how the rifle handles — it fundamentally changes what the rifle is under federal law. Welco …