gew98,
It's not bunk because some AR makers (not Colt) have offered "Match" barrels with the tighter, .223 Commercial "Bolt Action" spec chamber - having a much shorter throat than military spec. If you take a 5.56MM round, loaded to max pressure and with the bullet set out to the proper length for a military throat and chamber it in a .223 Commercial chambered rifle, the bullet may be forced into the rifling upon chambering, creating much higher than normal pressures. With the .223/5.56 cartridge, the result is sometimes a blown or pierced primer and gas released into the shooters face - not good for either the gun or the shooter! The specs are not quite the same, period. The good news is that the military 5.56MM chamber is in the vast majority of ARs - and it works with both ammo types just fine. But there is always the exception...... CC.