Look at any modern reloading manual. You will find separate data for .223 and for 5.56, and the 5.56 data is always hotter at the max end.
I'd say take another look and compare apples to apples . When you look at same bullet and powder . The max charges are about the same . With some 223 having higher charges . Assuming more powder means more pressure they seem to be closer then further apart in the manuals . At least the two I have that have separate loads for 223 and 5.56 ( Hornady & Sierra )
The thing I'm having a hard time believing is nobody from any test facility has put a factory NATO round in a test system that measures CUP . Or a 223 in a system that measures PSI . You would think those test would tell us once and for all if a factory NATO round produces more CUP then a 223 or vise versa .
Am I missing something here . It just seems like it should be relatively simple to figure out if 5.56 ammo is loaded to much higher pressures then 223 . If you test both 223 & 5.56 in both testing methods it should not matter that the tests are different .
You just do a blind test , no names just cartridge A produced xx,xxx CUP , cartridge B produced xx,xxx CUP . Then the same with "ALL" other test methods . Why would that not put this to bed once and for all ?
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