Heads up, Primary Arms has Spikes tactical 16 inch M4 carbine barrels in stock.
https://www.primaryarms.com/Spikes_Tactical_ST_16_M4_LE_5_56_SB51605_M4_p/sb51605-m4.htm
https://www.primaryarms.com/Spikes_Tactical_ST_16_M4_LE_5_56_SB51605_M4_p/sb51605-m4.htm