First, like alot of other things, not all carbon fiber barrels are equal.

Some are very high-tech, and others are simply a barrel liner with a carbon fiber tube sleeved over it.

Essentially, a properly made carbon fiber barrel will cool better than steel, weigh less than steel, and give good stiffness associated with the larger diameter but without the excess weight.

The actual accuracy will still be largely determined by the quality of the inner steel barrel and rifling.

The cooling of the carbon fiber barrel can lengthen barrel life in the throat erosion area.

Disclaimer:  I am the rep for Advanced Barrel Systems(ABS), which is probably the most high-tech carbon fiber barrel maker. They do make very nice barrels for AR15 type rifles, as well as bolt guns.

Here's a pic of a 16" AR15 barrel with a midlength gas system in carbon fiber  that shipped this week from ABS.
This particular barrel had a 100yd test target sent with it showing a sub 1/4" 3-shot group.

The barrel in the photo above has several features that contribute to its accuracy and barrel life.
In addition to the two forms of carbon fiber used, one type for winding and another type for thermal conductivity, that barrel has a Rock 5R SS match-grade barrel blank used for the inner barrel which is internally lined with a special Electroless Nickel/Silicon Carbide composite. Barrels using these types of construction have been found to last up to 85000 rounds in military test procedures. However, ABS has very conservative claims of over 20000 rounds of lifetime, even though many more rounds are likely to be expected from it.
For an ABS carbon fiber barrel that does not have the ElNi/SiC barrel lining material, the tested wear rate is 63% less than a chrome-lined military steel barrel, so it should last about twice as long as a chrome-lined barrel under similar shooting circumstances.

Many people feel that 1/4MOA is about the smallest size group that the AR15 platform is capable of, and this barrel actually does a bit better than that(not all shoot this well). It also is an HBAR contour that weighs about the same as a "pencil barrel", and has a computer-designed harmonic control pattern in the carbon windings that are CNC-wound onto the pre-profiled match grade barrel blank, and features pitch-based carbon microtubes mixed into the bonding matrix which gives an independent test-lab result of 372% better cooling than a comparable steel barrel, and has the  longest lasting interior lining material that is known at this time.

Quoted: Quoted: The barrel in the photo above has several features that contribute to its accuracy and barrel life. In addition to the two forms of carbon fiber used, one type for winding and another type for thermal conductivity, that barrel has a Rock 5R SS match-grade barrel blank used for the inner barrel which is internally lined with a special Electroless Nickel/Silicon Carbide composite. Barrels using these types of construction have been found to last up to 85000 rounds in military test procedures. However, ABS has very conservative claims of over 20000 rounds of lifetime, even though many more rounds are likely to be expected from it. For an ABS carbon fiber barrel that does not have the ElNi/SiC barrel lining material, the tested wear rate is 63% less than a chrome-lined military steel barrel, so it should last about twice as long as a chrome-lined barrel under similar shooting circumstances. Many people feel that 1/4MOA is about the smallest size group that the AR15 platform is capable of, and this barrel actually does a bit better than that(not all shoot this well). It also is an HBAR contour that weighs about the same as a "pencil barrel", and has a computer-designed harmonic control pattern in the carbon windings that are CNC-wound onto the pre-profiled match grade barrel blank, and features pitch-based carbon microtubes mixed into the bonding matrix which gives an independent test-lab result of 372% better cooling than a comparable steel barrel, and has the  longest lasting interior lining material that is known at this time. That really is incredible. So, what is it that makes the barrels so expensive to make? Is it the ElNi/SiC barrel lining?

Well, it is kind of a combination of everything.
But, it starts off with a top-notch match-grade SS barrel from Mike Rock, which would normally cost around $400 in a finished state, even if no carbon fiber or lining material was used.
So, it has a fairly expensive basis, before we even get into the exotic stuff.
Then, the carbon fiber materials are expensive, and the engineering and craftsmanship are expensive.
So, it all adds up.

Personally, I think ABS is selling the stuff too cheap.
When I was looking for a barrel like this, before I started working with ABS, I was expecting to pay over $1k for a good carbon fiber barrel. When I found out ABS was making barrels this high-tech for under $1k, then I knew I had to get involved with them.

Quoted: I'm really interested in this concept, but I'm not ready to buy one just yet. However I think you'll be hearing from me, later this year or early next. Will you guys be doing 1-7" twists at some point? What about mid-lengths? Just wondering .

We've always been doing 1-7" twists since the business began.
1-7" twist is one of our most popular orders,and the recent ARFcom group-buy were all 1-7" twist.

We do as many mid-lengths now as we do carbine lengths. No problem at all.

Quoted: That does seem impressive. Any anacdotal reports of them in FA use?

We've done one full-auto torture test on an ABS barrel and it went 600 rounds continuous full-auto without overheating enough for a cook-off.

We scheduled another one, but had a problem early in the test with an aluminum gas block failing, so that test was scrapped, and we haven't had another one.

We would like to do one on downloadable video, so everybody can watch it.
Maybe over the summer.

Quoted: You also offer PacNor blanks though, right?

Yes, we offer PacNor blanks if that is specified by the customer, and there is a $100 price increase associated with barrel blanks selected from other manufacturers than Rock, because it cost more to get those than the deal we have worked out with Rock.

Both Mike and I agree that we see no particular advantage to the PacNor polygon barrel, over the Rock 5R barrel. And the Rock 5R has proven itself as the "sniper rifle barrel of choice", whereas the PacNor has not. But, PacNor is a very good barrel, and we'll use that for a basis if the customer wants it. We've made a number of them, and they work out fine in the carbon fiber version, as well as standard.

I consider the Rock 5R the finest barrel blank made today.

Quoted: Are your ar15 barrels available now ? ( as pictured above ) Do you have a web page ?

Yes, they are available to be ordered.
We rarely have any barrels stocked, since custom barrels are the main line.
Mike is back at work and feeling better, and barrels are rolling off the line as we speak.

The ABS website is Advanced Barrel Systems
You can order by calling or emailing me at the info listed in my sig line below.

Quoted: Is there anything that can be done to make the SS more rust-resistant, or could this process be done to 4140,4150 or a CMV steel barrel?  I'm more worried about damaging my barrel due to rust than shooting it out.  Does the liner inside the barrel keep all moisture from getting underneath it to the SS or are their some kind of micro-cracks where moisture can get in over time?  If nothing can be done about the exposed SS on the outside, can something be done to darken it without hurting it's rust resistance?

4140, 4150, or CMV would be alot more prone to corrosion than any stainless steel would ever be.

However, yes the stainless steel can be covered with CeraKote or Dura-kote or something like that.

I live in East Tennessee, and we have a very humid climate here. I live in a log home and have no air conditioning. I've had my barrel for over a year now, and I've never done anything at all to protect it. There's not a spot anywhere on the barrel, inside or out, from any rust or corrosion occurring. It's just like new.

Rock barrels are 416 stainless, and they are very resistant to rust or corrosion from the elements.
I wouldn't worry much about that.
416 is even suitable for use in salt water marine environments.

I haven't seen any of them rust yet. Mine is still perfrect, and it sits in 90% humidity conditions most of the time. I have to really keep up with my blued guns, or they will rust away.

You can CeraKote or Duracoat the outer surfaces that are exposed stainless if you want.
The bore should be fine.
If you are worried, you can just run a lightly oiled patch thru it, and that would take care of any concerns.

The basic description of 416 SS is that it is a martensitic stainless steel that is not AS corrosion resistant as 316, but still resistant to basic salts and atmospheres, and is widely used in the food processing industry in wet processing environments.

Perhaps the austenitic steels are more corrosion resistant, but that is not necessarily construed as the only stainless steels that provide corrosion resistance.

Nowhere did I say that 416 was the MOST corrosion resistant stainless steel.
Perhaps I would have been more accurate saying that 416 is not the best  anti-corrosion choice for constant dunking into salt seawater.

I will say that 316 would be more corrosion resistant than 416, and that 416 would be more corrosion resistant than typical carbon steel used in barrel making. 416 still provides a good corrosion resistance, and is a good barrel steel.