Originally Posted By Unicorn:
Colt does not recognize a difference. Their chambers (and the military) have never changed. They were called .223 until NATO nations adopted it in 1980 or so. NATO did not create the caliber, they just started calling it their metric name.
It was originally designed as a military cartridge, with the longer leade and specific pressure.
Some company wanted a tighter match type chamber and this was the problem. It's been changed from the original .223 / 5.56 chamber designed. If the makers doing this were to have done itcorrectly they would have given it a totally different name.
Most commercial ammo is underloaded to avoid risk with these out of spec, improperly named chambers.
In reality NATO is using the Colt chamber.
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This. Bore diameter is .219 in which equals 5.56mm.