I've had an Armalite National Match rifle for a couple years now and haven't shot anything but .223 out of it.  I'm guessing that it's chambered for .223 since it's designed for accuracy, but checking the barrel would involve a lot more than i really want or am able to do.  I picked up some 5.56 surplus today for cheap, and i have other ARs to shoot it w/, but i'd like to know what kind of chamber my Armalite has.  thanks in advance!

I have an Armalite (T) version...similar barrel (Wilson Stainless, 1X8, yours 20", mine 24")

From the Armalite Website;

".223 Remington (SAAMI standard) and 5.56mm (NATO standard) rifle chambers are almost identical.  The difference is largely limited to the “freebore,” the cylindrical space in front of the case mouth, and the “lead” or “leade,” the the tapered region that eases the bullet into full engagement with the rifling.  NATO and SAAMI cartridges can normally be used interchangeably with no problem.

The SAAMI chamber features less freebore and a tighter leade, which normally provide better bullet fit and match-grade accuracy than the NATO chamber.  It is wonderfully suited to match bullets.  

Millions of rounds of NATO ammunition have been fired safely in Eagle Arms' and ArmaLite’s SAAMI chambers over the past 15 years.  Occasionally a non-standard round (of generally imported) ammunition will fit too tightly in the leade, and resistance to early bullet movement can cause elevated chamber pressures.  These pressures are revealed by overly flattened or powder stains that reveal gasses leaking around the primer.  

The first few rounds of ALL ammunition, from whatever source or lot, should be checked for pressure and other signs of defect before firing large quantities.  If you have a problem, you can generally bet that the ammunition meets neither SAAMI nor NATO specifications.

ArmaLite has adopted a practice of using a special, modified SAAMI chambers in its stainless steel match barrels.  This chamber is better for match use than the NATO chamber, but fires the NATO ammunition perfectly.  We use the NATO chamber in all moly (phosphated) and chrome-lined barrels."

You'll be fine...shoot it 'till it glows.

Armalite SS barrels use a Wylde chamber.  It will eat anything.  

223 Wylde Freebore = .0619 Leade angle = 1 deg, 15 min

thanks guys!  I had no idea Armalite used the Wylde chambers.