"The 6731 does not have a full chrome-lined bore, only chamber"
Is this the barrel that has to be cleaned after each shot while breaking in?
Also
I read that the 4150 non chromed lined bore is the longest lasting-

" Stainless barrels won't last as long, 4140 steel barrels wear longer than stainless; but not as long as the Colt 4150 barrels (w/o the chrome lined bores). nor as long as the chrome lined bores (given the same shooting). "

http://groups.msn.com/TheMarylandAR15ShootersSite/barrelfaq.msnw

I'm gonna wait to see if I can buy an LEO Colt M4 in Sept before I purchase an M4.
In the mean time I just purchased a Colt MT6731.
Any opinions and/or facts would be appreciated related to this model.
I know you usually do this B4 a purchase, but the price was good enough for an impulse buy.
Thanks in advance.

http://colt.com/CMCI/images/MTCompHBARII_BIG.jpg

I recently purchased a 6731-my first AR-and have been very pleased.  So far over 500 rounds of varied ammo and not a single stoppage or problem of any kind.  I also have been hovering around 2-3 inches at 100 yards with iron sights.  I'm no high-handed shot, but a friend of mine is and has consistently shot much smaller groups with this rifle.  I've no doubt about its' potential for accuracy.  

While new to ARs, I'm not new to firearms in general and collect older Colts and Smiths (pre-war or early post-war) for their fine fit and finish.  Any person who has been around firearms long enough, or has narrowed their interests to a particular category can recognize quality fit and finish and the 6731 has this in spades.

As far as barrel break-in, I've noticed few topics seem to generate as many varied and passionate resposes.  After reading a link to comments made by a custom barrel manufacturer, reading opinions here and research and reading on my own, I first cleaned my bore thoroughly with a bore snake and did so at an interval of every 20 rounds until the first 200.  Seemed like a resonable process to me anyway.

As to your question about longevity of the different barrels,  I'd bet that any differences-assuming reasonable use and maintenece-would be minimal.  Most quality modern firearms seem to be engineered in terms of material compostion to last as long or longer than the user-once again assuming reasonable use and maintenence. I realize that some shooters actually DO manage to wear them out, but I think most folks don't shoot 10, 20 or 30,000 rounds through their firearms as these users do.

Even without the major advances in metallurgy immediately following WWII, older Colts and Smiths-if not abused and well maintained-will most likely outlast the user- and that's without the benifit of modern-day steel.  Some of mine have seen more than their share of use, and I have no doubt that I'll not wear them out any more than the previous owners did.

From what I've read online, in books and from conversations with folks a lot more qualified than I to discuss such things,  modern day chrome-moly steel, due to advancements in heat-treating and  composition is of such a vastly higher quality than the materials used before the war that it would seem to take a LOT of use and abuse to wear out a quality mordern firearm.

Just my opinion.  I'm sure others will disagree-but that's the whole reason I enjoy forums like this.  I always end up learning a lot more than I have to pass along.