I am spec'ing out a new AR to build and would very much like it to be a middy.  Armalite has nice middy barrels, but they are listed on the website as .223 barrels.  Does this mean I could not fire 5.56 ammo?

If not, who makes a good quality mid-length 5.56 chrome-lined barrel?

Read about the differences here then you can better decide.

Quoted: Read about the differences here then you can better decide.

+ a big one

Armalite stainless steel match barrels are .223

All armalite moly or chrome lined steel barrels are 5.56mm

Never mind... I found the answer.  This is directly off the Tech Notes section of armalite.com...

TECHNICAL NOTE 45:  5.56 NATO vs SAAMI .223 REMINGTON CHAMBERS    

BACKGROUND: We are often asked whether our rifles feature NATO (North Atlantic Treaty Organization) or SAAMI (Small Arms and Ammunition Manufacturers Institute) standard chambers, and whether it makes any difference.  

FACTS:

.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.

ArmaLite’s larger AR-10® rifles are all chambered with 7.62mm NATO chambers.  .308 Winchester (SAAMI standard) ammunition functions perfectly in the 7.62mm chambers.