The 556 barrel is very high quality. There is less need for a chrome lining when you don't have a DI gas system, and the Sig barrels go through nitrocarburizing process that leaves them very durable in their own right. There are all sorts of heated internet tough guy debates about the advantages and disadvantages of each type of barrel, but the Sig is hardly pot metal.
I've read barrel life is something like 15k rounds under heavy (like, military) use and more like 30k for a recreational user.
It's not quite the same as the early M16's that lacked a chrome lining to save a few cents per gun; it uses a different metalworking process that is very good in its own right, and may even posses some minuscule accuracy advantages over chrome in addition to being somewhat less sensitive to harsh cleaning chemicals than chrome.
