Arms Unlimited has them back in stock.
https://www.armsunlimited.com/Colt-M4-14-5-inch-556mm-SOCOM-Barrel-Assembly-p/sp65054.htm
https://www.armsunlimited.com/Colt-M4-14-5-inch-556mm-SOCOM-Barrel-Assembly-p/sp65054.htm