Short-barreled rifles (SBRs) are one of the most popular ways to build a compact, maneuverable AR-15. A barrel under 16 inches makes the rifle easier to handle in tight spaces, easier to store, and easier to carry in a vehicle. But a shorter barrel changes how ammunition behaves, and not every cartridge performs well when you cut the barrel down. If you're planning an AR-15 SBR, caliber is the first decision that shapes everything else: your gas system, buffer, ammunition cost, recoil, noise, an …