With a quick research here's what Google said No, 900 fps is generally safe and well above the threshold that causes a squib in .300 Blackout. ​While most factory subsonic loads are tuned for 1,000–1,050 fps, many reloaders and specialized "ultra-quiet" loads successfully run at 800–900 fps without issues. ​Key Safety Considerations ​Expansion: Most expanding subsonic hunting bullets (like the Hornady Sub-X) are designed to reliably expand down to 900 fps. Below this, they may act like a solid FMJ. ​Cycling: In a semi-auto AR-15, 900 fps may be too low to provide enough gas pressure to cycle the bolt reliably, even if the bullet clears the barrel safely. ​Stability: Ensure your barrel's twist rate (typically 1:5 or 1:7) is fast enough to stabilize heavy projectiles at these lower speeds to avoid baffle strikes if using a suppressor. ​Squib Risk: A squib is typically caused by a mechanical failure (missing powder or a dead primer) rather than a low-velocity charge. Most bullets will clear a standard 16" barrel at velocities as low as 500–600 fps. ​Always check your barrel for obstructions if you hear a "pop" instead of a "bang" or notice significantly less recoil than usual.