Allegiance Ammunition has an interesting product, their SilentStrike .223. SilentStrike purports to be the worlds first fully functional sub-sonic .223 round. The bullet is 100 grains and in the video, Allegiance claims it will work in the AR-15, M4, and HK-416. Down a 16″ barrel, the round is reported to be at 975 fps velocity. This equates to about 200 ft/lbs, or about half of what 9mm typically provides in 4″ barrels.
No word if you need a specific length gas system to get the weapon functional like a 300 BLK.
Description from Allegiance Ammunition:
Whether a law enforcement or military team that needs astonishing stealth combined with maximum power and performance, SilentStrike delivers. SilentStrike fully functions in semi and full auto the M4, AR, weapons platform at a silent 975 fps. No modification to the weapon is needed. SilentStrike is currently produced in a lead free fragmenting Tungsten powder core. The fragmenting SilentStrike gives excellent results in soft tissue even at subsonic velocities.