The primary difference between AR-15, AR-9, and AR-10 triggers lies in the hammer profile and mass, which are designed to match the characteristics of each platform and its operating system.
AR-15 triggers are designed for standard intermediate-caliber platforms (e.g., 5.56/.223) and use a hammer profile suited for typical gas-operated systems.
AR-10 triggers feature a heavier hammer profile to reliably engage the larger and heavier bolt carrier group found in .308/7.62 platforms.
AR-9 triggers are designed for blowback-operated systems, which have different cycling characteristics and require a hammer profile optimized for proper function and reliability.
While the trigger housing and pin dimensions are often similar, the hammer design is the key factor that ensures proper performance across each platform.