That's not the issue at all... The 5.56mm NATO or mil-spec chamber has a longer leade than a .223 or SAAMI chamber. The SAAMI chamber is only required to be proof tested to the lower SAAMI chamber pressures.
And 5.56 and .223 brass are not the same (internally)... Externally they are almost the same (couple thou difference) but the 5.56 brass is thicker (less case volume) so don't drop your .223 powder charge into a 5.56 case and expect it to be safe, 'cause it likely won't be...
Don't go by what's stamped on the lower receiver... it doesn't indicate what chamber the barrel has cut in it. Go with what's stamped on the barrel itself to determine what chamber a given rifle has.
SAAMI themselves regard 5.56 in a .223 chamber as an "unsafe arms and ammunition combination". See their web site at:
http://www.saami.org/Unsafe_Combinations.cfm (scroll down to centerfire rifle)