Nordic Comp. has has a nicely priced 16":
Wilson-Nordic AR Rifle Barrel, Stainless 16"
Here is a link to the White Oak Arms barrels Sean is referring to:
WOA 16"
You can get the 18" with a mid or rifle length gas system - WOA 18"
I have the 18" WOA barrel with the rifle length gas. Still breaking it in, but I really like the weight. I really prefer the recoil impulse of the rifle gas vs. the mid gas.