User blog comment:Master Moridin/Mercury's Boots Don't Just Shoot Bullets/@comment-26397825-20151210053905

That's a good theory, makes sense, and it fits with what we've seen so far. Also explains how he doesn't fire every time he takes a step, and how he reloads.

Firing require him to actively use his aura to do so.

Reloading is not a problem because he has dust in there, which can be used many times before running out. Certainly much more than a magazine can. He just needs to top off his dust supply at the end of a fight.