A gunsmith friend tells me there are at least one dozen different 5.56mm/.223 chambers. Here are some: Click on reamer dimensions:
http://ar15barrels.com/tech.shtml
Quote:
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Commercial .223 brass can be thinner than 5.56 NATO brass. The NATO brass may have a smaller volume than .223 brass.
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Since 1968 i've been weighing the cases for my accuracy loads. Some British made 5.56mm military brass is very thick. US military 5.56mm brass is not heavier/thicker than .223 made commercial brass. The heaviest US made cases are Federal Gold Medal. The heaviest cases of all are those made by Lapua.
Click on brass weights:
http://ar15barrels.com/tech.shtml
This young man did some fine research:
http://www.luckygunner.com/labs/5-56-vs-223/
Since 1968 i have fired at least a half million rounds of US military M193 5.56mm ammunition in the .223 chambers of numerous guns with no ill effects. Some of my guns have been re-barreled five or more times. i would not attempt to fire a US military 5.56mm round in a .223 match chamber.
i'm not suggesting that you fire 5.56mm ammunition in the chamber of your .223 rifle.