Best lightweight barrel on the market. - Faxon Firearms AR-15 5.56 NATO Pencil Profile Threaded Barrels

This is easily the best lightweight barrel on the market for a number of reasons.

1) Weight - at 19 oz for a 16" barrel, nobody else sells a lighter one.
2) Price - usually runs around $150 to $175, depending on if you catch a sale or not, which is an excellent deal for a barrel at this level of quality.
3) Accuracy - ultra-light barrels have a reputation for poor accuracy. Both my barrel and a friend of mine who has the same barrel hang in the 2.5 MOA camp with cheap-o FMJ ammo, and can get down into the 1.5 MOA range with match grade ammo. That's not benchrest accuracy, but it's plenty good for an AR15 that you're not shooting 500 yard matches with.
4) Consistency - ultra-light barrels have a reputation for a wandering zero once they get hot. This one does not seem to share that issue - point of impact does not wander as it heats, at least not enough for me to notice.
5) Accuracy when hot - most ultra-light barrels get pretty bad for accuracy when you get them really hot. This one does open up a bit if you get it scorching hot, but just a bit, nothing overly dramatic.

As with all pencil barrels, it does get hot FAST, so probably not the best choice if you do a lot of mag dumps, but it does also cool off much faster than a heavier profile, so if you do get it disturbingly hot now and then, you won't have to wait too long for it to cool back down.

All in all, highly recommended. This is the best barrel on the market if you want to build a sub-6lb gun. With a little creativity and some polymer parts, you can build a sub-5 or even sub-4lb gun using one of these.