This is the correct flip Aperture for all Colt models starting with the M16A2 onward. This would include all versions of the M4 as well as "removable" rear sight assemblies that use a M16A2 style aperture.
The M16A2 Aperture has a larger diameter hole for 0-200 meters, whereas the M16 (M16A1) used the same diameter hole for both short and long range positions.
All Flip Apertures (AR15, M16, M16A1, M16A2, M4, C7, etc.) are interchangeable with each other since they all have the same external dimensions... meaning you can upgrade an earlier rifle with this easier to use later style Aperture.
If you are looking for the earlier M16 Aperture (Here)
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