Short of any long duration full-auto fire, that barrel (assuming it's chrome lined) should last 20,000 - 25,0000 rounds.

Don't over clean and it will last a long long time.... I have some with over 50,000 rounds and I've always cleaned to moderation.
Cleaning does more damage than shooting....

Stainless Steel is my choice for longevity but there are only a few around....

Dave S

One of the gun mags did an endurance test on a Colt about ten years ago, and managed to shoot through its best accuracy in just under 10,000 rounds of aimed semi-auto fire. They fired all 10k in like one or at most two days, though.

IIRC, the groups started opening up in the mid 9000s.

Dave (Spec Ops) and John (Forever) bring up good points, and I would like to share some things I've learned
over the years, and some I've only recently learned.

CMMG does make and sell very good products, but to give you a direct answer to the question of barrel life is difficult
because there are so many variables that can determine how long any barrel will last. Carbine asks specifically about
barrels chambered for the 5.56x45 caliber (.223 Remington), this gives any barrel a head start on longevity because it
is a sweet, well mannered, and supremely flexible cartridge design that is not known for barrel burning. Had the question been
about a magnum, the life expectancy would be drastically reduced. The 5.56 doesn't erode the throat as quickly as the hotter
cartridges do, and as we all know, this is where most of the life shortening damage occurs.

Next up is what one defines as life expectancy. The answer revolves around quality of life, or to be exact, how well do you expect
the barrel to shoot as it ages. I think there are 2 types of accuracy: Practical (every day sporting use, 1 to 2 MOA), and Competition
(below 1 MOA). A shooter who has no real need for Competition Accuracy will not think he's in need of a new barrel until it is
really shot out. Whereas a competitor may want to replace his barrel at the first sign of his groups opening up. Practical will see
10K to 25K from a quality rack grade Cro-moly barrel. Competition will probably end up being used as a tomato steak around
2K to 5K.

The powder you use in your loads also has a bearing on longevity. Some powder burns hotter than others and will accelerate throat
erosion. Spherical powders (ball) will extend barrel life, while double-based extruded powders vaporize metal like a laser. Then there is bullet weight,
shoot tons of heavy 75-77 grain loads and life expectancy declines because they have heavy powder throw weights.

Next up is material, and as Dave S stated, Stainless Steel is a good choice for longevity. We have two choices, essentially, SS and Chromemoly
Steel. SS dies a very gradual death with almost undetectable widening of groups; Chromemoly can and will die all of a sudden. Both materials
will shoot the same, and are accurate, but each reacts differently to that gas jet that hits the throat. There is also a steel specific to machine guns,
which CMMG doesn't use for its rack grade barrels, that will out last most materials. Barrel steel types have alloy numbers like 41v45, that refer to
the specific alloy content in the steel. So to review; Chromemoly will shoot better longer and die suddenly, SS dies a slow gradual death.

All is not lost. If there is enough material in your barrel, it is possible a competent gunsmith can perform a set-back to bring new life to your dead
barrel. Sometimes the breech end can be shortened by cutting and then re-chambered. A new extension is fitted, headspaced, and you are
back shooting again. But don't expect this to give your barrel the same life span as it did when new, all you've bought is some time.

One last thing: NO, the type of rifling has no direct contribution on barrel longevity if it was done with care. A button rifled barrel won't have any greater
life than a single cut rifled barrel, however, a cold hammer forged barrel may shoot a bit better because the muzzle end bore diameter can be slightly
smaller in diameter than the rest of the bore. This constriction helps seal the bullet and more uniformly shapes the exiting gas jet as it passes the crown.

Ted

My buddy has a CMMG with piston system. Non chrome and he got around 60,000 rounds out of it before he started seeing any accuracy degradation. He is a very, VERY good shooter, a former Army Ranger (for real not one of those asshats that claim to be). He now has almost 80,000 rounds through it and it still shoots ok at 200y. He is not a big fan of over cleaning his guns, he didn't use more than a bore snake on it until around 15,000 rounds. He did however clean the bolt and other stuff every time he took it out. Sold me on their barrels.

Although I said I'd never return to this forum I couldn't help but at least try to defend myself.

Fatal, you are correct based on the way I wrote the post, in fact everything you wrote is right on the money when you
take that one sentence verbatum. However, if I may re-state by saying this:

While the powder throw weights for the heavier competition bullets are LESS than the grains associated with lighter grain
weight bullets, there are heavy bullet loads with powder throw weights that are VERY HIGH for .223 Rem catridges using
heavy bullets.

Of these I refer to the Proven Load ID# 1771, Berger VLD 80gr., 26.5 gr. Win. 760; or Proven Load ID# 6531, Sierra
MatchKing 80gr, 25.5 to 26.5 Hodgdon BL-C(2); or Proven ID# 11176 Sierra MatchKing 90gr, 24.8-25.3grs. Federal
GM205M; or even the Unproven Loads like ID#4937 75gr Berger VLD, 26gr of Varget.

As opposed to heavy bullet loads developed using less powder like Proven Load ID#6532, Sierra MatchKing 80 gr., 23gr
of Hodgdon H4895, or Proven Load ID# 6533 (same bullet, I think), 24gr Hodgdon H4895; Compare these to the load that
best typifies your assertion, such as Proven Load ID# 9905 using an 80gr bullet and only 18.8grs. of Vihtavouri N135.

So yes, in comparison to the 55gr. bullet cartridges they use LESS; HOWEVER, my point remains the same, heavier bullet
cartridges - especially those with throw weights high for the class - will burn a barrel out faster than the lighter bullet loads.

Now before you go ballistic, there is another side to the story in that it isn't the fault of the powder or even so much as the weight of
the load, AS IT IS THE TIME THE GAS JET IS AT ITS HOTTEST & CLOSEST TO THE BULLET AS IT TRIES TO GET THE HEAVY
BULLET MOVING. True, the amount of time is in milliseconds, but it still takes longer to launch the heavier bullet, and that will vaporize
metal faster than the powder rich, low grain weight bullets. The little guys take less time to get moving, the flash is less confined for a shorter time,
therefore it causes less damage.

As you can see this took considerable space to explain, and like it or not, every word is correct in this revised context, just as you are when taking
the orginal sentence, and me, to task.

Ted