I'm not an attorney, but I do have some experience with patents.
My recommendation would be to do some research on the paticular magazine you want to copy. US patent laws require a manufacturer to mark a product as patented, so I would review the gun and magazine to see if it's marked with a patent number or patent pending. Next, I would do a quick review of their literature (specs sheets, catalog page, etc) for any patent information. If the manufacturer fails to mark thier product, you stand a much better chance of limiting damages if you were to violate a patent. Finally visit the US patent and trademark office and search the patent database to see if the manufacturer has any listed patents. Searching the USPTO website can be tedius.
Swamp Yankee is correct in his descriptions of Utility Patent and Design Patent. Design patents cover the ornamental design, while a utility patent addresses the functionality. Utility patents include a number of claims, which are very specific elements that define the scope or limitations of the patent.
My guess is that since magazines have been around for such a long time, any patents that may exist on a paticular magazine will be very narrow in scope (improved follower, means of attaching the base cap, etc). Typically, the more narrow the scope of the claims, the easier it will be to design around the patent. If you were to find a patent on the paticular magazine you want to produce, a patent attorney would be able to offer guidance on whether your design violates the patent claims.