Ammunition specifications can vary by cartridge designation, projectile weight, construction, and intended application, making firearm compatibility an important consideration before use. Subsonic loads are generally designed to operate at velocities below the speed of sound, while full-metal-jacket construction describes a projectile with a metal covering. The phrase 300 AAC Blackout 200gr subsonic FMJ ammo identifies a specific combination of cartridge, projectile weight, velocity category, and construction. Firearm owners should verify chamber markings and manufacturer guidance before handling ammunition and follow applicable laws and established practices for storage, transportation, and safe handling.