Quote Originally Posted by nun2sharp View Post
The.223 caliber rd is the same as the 5.56 mm rd, the difference being whether you measure it by the inch standard or the metric.
5.56 is actually a little bigger. You can notice when you groups go to heck with just switching from 5.56 round to .223 in a 5.56 barrel. The bullet will actually wobble on a very small scale. I've only seen this with my uncle's 5.56 but they say it common on some of the gun forums.

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Devilpup