I was reading about the Sig 550 on biggerhammer.net. They have a section which details the torture testing that the Sig 550 went through before it was adopted. It also gives some impressive statistics on estimated number of rounds before failure. Check it out at: http://www.biggerhammer.net/sigamt/550/550techinspection/

My question is how does this stack up against the AR-15? How many rounds is the AR designed to last? Is there a similar document detailing the tortures that the AR can withstand? Doesn't NATO have a standard that each member nations small arms have to meet?

While I can't profess to know all of the thing you ask, especially pertaining to NATO, this is what I recall:

The M-16's barrel MAY last up to 30,000 rounds. This is of course provided that you're not expecting benchrest shooters accuracy and it is maintained [cleaned] properly.

The things which would effect the life of the barrel are the cleaning, as mentioned, as well as thermal cycling. The chrome lining is supposed to give the M-16 the benefits of corrosion resistance and added life.

There are barrels on many firearms, The M-16 and others, which may end up being changed sooner than 30K rounds mostly due to poor maintainance.

As far as NATO goes, I don't know if they have any requirments or what they may be.

I can't make a good comparison between the 550 and the M-16 as I don't have enough info on the 550.

Ok, maybe it would help if I posted some of the numbers:  

2.- Weapon service life
2.1 Service life      of individual components
The service life or      individual components is classified into 4 groups as follows:
a) min. 10000 rounds        for wearing parts of the bolt assembly
b) min. 15000 rounds barrel life
c) min. 15000 rounds for all other secondary components
d) min. 30000 rounds for major components.
2.2 Extended service life / Functional safety
- Experience indicates        that depending on the grade of ammunition used, the barrel has to be changed after approx. 20000 up to 30000 rounds have been fired.
-15000 rounds for        wearing parts of the bolt assembly are quite feasible: other small components        will last for anything up to 30000 rounds.
-Major components        such as trigger housing, bolt, receiver, plastic parts, the main components        of the trigger action may readily be expected to withstand 40000 rounds.
-We have weapons at        our plant which have major components still fully functional after firing        80000 up to 100000 rounds.

They are not likely to be among the cheapest in their category, but these are  comprehensively designed weapons. In addition to offering all of the functions  a modern assault rifle is expected to perform, at a high quality/price ratio,  they have an estimated service life of 30 years - which speaks volumes for their  intrinsic  sturdiness.

I find these numbers pretty impressive and was just curious how the AR-15/M-16 compared. I know there has to be data and testing done somewhere but maybe the government hasn't released it. 30,000 rounds for the barrel life sounds pretty good though.