The short answer to this is No. The AR-15 CompMag is a single stacked design which will only accommodate .223/5.56 ammunition. We are currently working on an AK47 version that we hope to have out in late July or August. Other versions such as .308 (AR10) will be manufactured in the future, but will not be available until much later. We will put updates on our site as we begin new versions of the CompMag.
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 the AR-15 CompMag for use with .300 Blackout ammunition.