The current versions of the AR-15 CompMag (GEN.3 and GEN.2 sold after Jan.31 2019) can accept .223, 5.56, .300ACC Blackout, and .277 Wolverine ammunition.
The AR-15 CompMag will not accept 6.5 Gendel, 6.8 SPC, .458 SOCOM, .50 Beowulf, 5.7x28mm, or 7.62x39mm.
The AR-10 CompMag is compatible with .308, 7.62x51, and 6.5 Creedmoor ammunition.
The AK-47 CompMag will only accept 7.62x39.
For AR-15 CompMags bought before 2019 the following applies.
The AR-15 CompMag can accept .223/5.56 ammunition.
The AR-15 CompMag will not accept 6.5 Gendel, 6.8 SPC, .458 SOCOM, .50 Beowulf, 5.7x28mm, or 7.62x39mm
Although this is the case, the AR-15 CompMag can be easily modified to work with .300 Blackout ammunition because the diameter of the case is the same as the .233/5.56.
Below is a picture tutorial instructing you how to do this.
How to modify earlier versions of the AR-15 CompMag for use with .300 Blackout ammunition.