Here is what ArmaLite has to say about 5.56 Nato chambers and .223 chambers:
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.
Gaillard in Sask. has 8 twist barrels and just about any gunsmith can chamber for .223.