i have a brand new never fired ESS solutions m4 barrel 16" with 5.56 chamber. these barrels can be used to make a 16" m4 ar upper or a "dissie" style upper.
you can even do a colt 605 replica from one of the many a1 kits out there.
this barrel has .063 gas port , 4140 sae steel, 1-7" twist with 1/2-28 on the muzzle. $95 shipped. i can open up the port to .076 to run crap ammo like tula for no charge if you ask before shipment.