The engineer in me can't see it. Why take a brilliant versatile cartridge like 5.56mm and neuter it with a short barrel, and then pay the government for the privilege? If you need short, get a AK folder or a Galil/R1, SCAR, Tavor or AUG. Seriously even 16" is marginal. 18" better. Even the 300 BLK is a better, more versatile weapon with 16 inches. I just don't get it?