I was looking at the picture of Optimus' gun on my desktop wallpaper, also as seen in the picture I've included in this post. I began to look past the obvious hugeness and coolness of the design and realized just how well imagined it is. If you take a look at it, you see the components of a combustion device. A piston, a gasket, an exhaust vent... The movie designers imagined an advanced technology that is plausible given the mysterious and powerful energy sources which Transformers in the movieverse run on. Not only is it brilliantly imagined, it takes into consideration the advanced nature of the alien who possesses it. Instead of giving Optimus a gun that requires an ignitable such as gunpowder plus cartridges or shells, they give him a gun that is part of him and uses a combustion mechanism powered by his own energy to achieve the necessary pressure to create the propulsion of his metallic rounds. He has no need for manufactured bullets, only hunks of metal that can be blasted utilizing the combustion chamber in his gun. It would also allow him to vary the velocity of the slugs completely at will! I mean, this is a cool idea. I just had never studied it before. I wanted to give props to the concept artists who worked on this and other great ideas - say what you want about the Bayverse, but these guys are not part of the problems with it.