There are a few more parts that have to be made of metal depending on the type of action used by the firearm. At a minimum, you'd also need a metal bolt/breach

There are a few more parts that have to be made of metal depending on the type of action used by the firearm. At a minimum, you'd also need a metal bolt/breach face (the part that supports the rear of the cartridge) for any type of action. I'm also fairly confident that any springs used (the firing pin/striker/hammer needs one unless you want to fire it with a handheld mallet) in the design would have to be made of metal as well unless there are some composites I'm not aware of that match the properties of spring steel. Adding that metal to a firearm doesn't add that much mass but those parts would increase the profile of the gun due to their size/shape/area covered in the eyes of a image sorting algorithm.