The AR-15 platform is famous for one thing above almost all else: versatility. Unlike many rifle designs that are locked into a single chambering, the AR-15's modular architecture lets shooters swap uppers, barrels, and bolts to run a wide range of calibers from the same lower receiver. Whether you're building a first rifle, adding a specialized upper to an existing setup, or just trying to understand what all the caliber talk online actually means, this guide breaks down the most popular AR-15 …