You've decided to build your own AR-15. Smart move. Building your own rifle gives you a deep understanding of your firearm, lets you customize every component to your needs, and — depending on how wisely you shop — can save you real money over buying a comparable factory-built rifle. But here's where most first-time builders hit a wall: not all AR builder kits are created equal. A kit listed at $299 and a kit listed at $699 can look nearly identical on a product page, yet the gap in …