Bulky power sources at this point in the timeline are entirely consistent with the book. What's wildly inconsistent is that Cleon has a personal force field that's supposed to be invented a hundred years later, on Terminus, isolated from the Empire.

Additionally, since the audience needs to "see" the beam, most sci-fi weapons have miserably slow firing rates, enough that on the modern battlefield they would be useless.

"Guns in general seem like something that should already be antiquated."

This comes from a mindset that technology is "tiered" and that later technologies are "better" than earlier ones in all ways. This is caused by watching too much science fiction and/or playing too many video games. It is not how the real universe works. In reality, throwing things really hard at your opponent is likely to be a viable strategy indefinitely. The science fiction technologies that would invalidate this, like Dune's shields, do not seem to be things that exist in the real universe.