9mm barrels are designed for the 9mm Parabellum cartridge, a widely popular handgun round known for its efficiency and low recoil. In the context of AR-style rifles and carbines, these barrels allow for lightweight, compact firearm configurations, ideal for close-quarters shooting and personal defense. The 9mm barrel typically features a smooth bore optimized for the pistol-caliber projectile, ensuring reliable accuracy and performance. Due to the round's popularity, 9mm barrels and compatible firearms offer significant cost savings in ammunition, making them a preferred choice for frequent shooters and training applications. Additionally, the compatibility of 9mm barrels with standard AR-15 lower receivers, using appropriate adapters or dedicated setups, enhances their appeal by providing versatility and ease of integration into a shooter’s arsenal.