Quoted:
The PDF of the Crane test says M4A1, which presumably has the "SOCOM" barrel that is heavier under the HGs than the M16A2's govt profile barrel (though I couldn't find anything to confirm or refute that in the file)
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If you look in the file, there are pictures of the barrels that were tested. The tested M4 barrel was a normal M4 profile and not the "Heavy" M4 profile currently in use.