These are two different rounds. There is some good information out about this. Check out the recent articles in SWAT by Ned Christiansen with regard to this. Also, a fair number of makes mark their chambers 5.56x45 but are not correct. When I was at the Dean Caputo class in Four Oaks NC last fall, more than a few chambers were measured then reamed with Ned's chamber reamer; all needed it badly. There are a couple of places you can get "the chart" by Rob_S, like m4carbine.net.

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NHI, 10-8.