User blog comment:FairyRaven/Fairy Tail Chapter 391: Gray vs. Silver, Review and Commentary/@comment-4933386-20140705051946/@comment-4829046-20140705104552

I think he set the cannons to automatic where there is a conveyor-belt or suction effect that draws up rubble. I've seen that kind of thing before in anime and the user didn't manually load the rubble, the weapon did the loading by itself.