Choosing a caliber for an AR-pattern build starts with your real use cases—where you practice, the distances you shoot, and how you prioritize controllability, parts compatibility, and overall cost. Two routes that often come up are intermediate rifle cartridges and traditional pistol cartridges. While they can live on similar receivers, they deliver very different shooting experiences. Understanding the strengths and compromises of each helps you pick components that support consistent training and reliable performance, rather than chasing specs that won’t matter on your range days.