What is the difference between a patent and a copyright?

Both.  You need a patent to protect your property  interests in your weapon design, and a copyright to protect your property interests in your comic book.  A patent issued by the U.S. Patent and Trademark Office is the grant of a property right to an inventor – i.e., "the right to exclude others from making, using, offering for sale, or selling" the invention in the United States or "importing" the invention into the United States.  A person who “invents or discovers any new and useful process, machine, manufacture, or composition of matter, or any new and useful improvement thereof, may obtain a patent.”   A copyright, on the other hand, protects the legal interests of authors of “original works of authorship,” including literary, dramatic, musical, artistic, and certain other intellectual works, both published and unpublished.

However, the copyright protects only the form of expression, not the subject matter of the writing.  For example, a description of your weapon could be copyrighted, but this would only prevent others from copying your description.  You need the patent to prevent others from writing a description of their own or from making and using the weapon.   You should seek the services of an experienced intellectual property attorney to help you navigate, in particular, the patent application process.  Although the basic copyright application process is much more straightforward, because your comic book contains the design you need to patent, it presents a complex situation best handled by a legal professional.