Scraby hit the nail on the head. You don't want the cans stuck in the way when you're trying to point the gun at ememies. The animation of the cans loading into the gun would distract your attention from movement in the distance, like enemies. So, have the cans load at the bottom so they don't get in the way of aiming or distract attention.
Plus, the bottom one has the more obvious handgun shape, just looks much sleeker
