Heck, it you want a chrome lined barrel Oly will even sell you one of those.
I live in the Pacific North West and I shoot in the rain and my non-chrome lined chrome-moly barrel has yet to rust out like a Yugo in the mid-east.
Natty, what do you want out of your AR15.
Mine is accurate (5 shots using mil-surplus ammo into a 3 inch group at 200 yards with iron sights).
If you are concerned about mil-spec'ness (of a non-Military rifle), you want a barrel that is going to last longer or you want one that you don't have to clean all the time then get a chrome lined barrel and chamber.
If you want accuracy then chrome lined (and flash hiders, muzzle brakes and any other shit hanging off the end of your barrel) is out, as is a 16 inch barrel.
The fucking morons who say they want accuracy shouldn't be commenting on a 16 inch non-chome lined bore other than to tell you what they use.
Bunch of Oly bashers, this is how they get their rocks off.
In the end it is up to you!
From Oly:
CALIBER
First off, every barrel is stamped with the caliber.
The standard .223 caliber rifles are marked "5.56"
(Which they are not, they are marked 556)
STAINLESS STEEL BARRELS
If after the "5.56" markings, if you also see "SS", then your barrel is manufactured from 416 stainless steel. This material is very resistant to corrosion, and is universally accepted as one of the best barrel materials available.
CHROMEMOLY BARRELS
If the only barrel stamping is "5.56", then the barrel is made of 4140 chromemoly steel. Chromemoly is a type steel that incorporates chrome right into the barrel metal itself. This barrel material is corrosion resistant, but not as much so as a stainless material. Using the 4140 chromemoly material allows us to give the consumer a barrel with similar benefits of a chrome lined barrel, but maintain consistencies in the bore that allow for superior accuracy.