.223 Rem. (5.56 x 45 NATO) Subsonic

The .223 Remington (5.56x45mm NATO) is a widely used cartridge renowned for its versatility, but it presents notable challenges when tailored for subsonic applications. Designed originally for high-velocity performance, the .223’s relatively small case volume and light bullet weights make subsonic loading more complex compared to calibers specifically engineered for subsonic use like the 300 Blackout. The small bore diameter combined with lower velocity limits energy transfer and terminal performance, especially when using standard FMJ bullets.

For effective subsonic .223 Rem loads, reloaders typically use the heaviest bullets available—usually in the 75–90 grain range—to maintain stability and deliver enough mass for energy retention. Match-grade bullets like the 77-grain Sierra MatchKing or 90-grain Berger VLD are common choices. Twist rates of 1:7 or faster are preferred to stabilize these heavy projectiles, especially at subsonic speeds around 950–1,050 fps. Suppressed shooting is virtually essential for reliability and performance, as it helps improve backpressure for cycling in semi-automatic rifles.

Popular powders for subsonic .223 Rem loads include Trail Boss, Hodgdon CFE BLK, and IMR 4198, though reliable cycling in gas-operated rifles often remains inconsistent. As a result, many users opt for bolt-action rifles or tune their AR-15s with adjustable gas blocks or heavier buffers to improve reliability. In short, while subsonic .223 Rem is achievable, it is best suited for suppressed bolt-actions or highly tuned AR platforms and is generally considered less effective than calibers specifically optimized for subsonic shooting.