When I saw The Force Awakens, I had some doubts about the new Stormtrooper design, but the variant I liked the most was the 'Heavy Gunner' (see image below).
However, what I quickly noticed was that the things on his chest, which I had assumed were spare magazines, didn't seem to match up with any of the guns the Stormtroopers use. Plus, the boxes are so smooth, I don't see how they could be used for ammunition unless the important bits are facing downwards (and even then, how do you grip them?).
So, since this question about Chewbacca's chest boxes got an answer, I thought I'd ask about these chest boxes here. What are they, what are they used for, if they are magazines what guns do they fit into?