Its fairly obvious from the footage that the 3GL's chamber...

Its fairly obvious from the footage that the 3GL's chamber slides forward and in no way impedes the M16s magazine, it doesn't get in the way at all. Would you honestly feel that months if not years of design would overlook such functionality??

The video also has a caption saying the rifle's magazine can be inserted normally so in all probability it is off so that the video can demonstrate that there is no impediment to loading the rifle with the 3GL attached.

It's a winner throughout guys.