So I know that M16 have been marked .223 even though the chamber was actually 5.56. My question would be when what model was the first to have a 5.56 CHAMBER.
Did the XM16E1 have a .223 or 5.56 chamber.

Thanks

All M16's are chambered for 5.56. Even the 601.

Interesting. any one know when the 5.56 was derived from the .223?

Actually, it appears that some 601/602 chambers were tighter than the original SAAMI specs for .223 Remington.  Remington and Colt were clearly not comparing notes when Remington tweaked the specs of their .223 ammunition in preparation for its commercial introduction.  This is why the late 1962/early1963 military evaluations showed so many pressure issues.  There was such a mismatch in dimensions that bullets were even getting stuck in the rifling leade when unfired cartridges were extracted from the chamber.

FWIW:  The original USAF designation for .223 Remington was 5.64mm.  It wasn't until the Army took over responsibility for the ammunition that it was redesignated 5.56mm