So you've decided to join the growing ranks of 9mm AR enthusiasts — great choice. The AR9 platform combines the familiar AR-15 ergonomics you love with the soft-shooting, affordable-to-feed characteristics of a pistol-caliber carbine. But now you're staring at a dozen browser tabs full of parts, wondering how any of it fits together. Here's the uncomfortable truth that most forum threads bury on page three: building an AR9 from random parts is a genuinely risky proposition for beginners &m …