They aren't plastic. They are polymer. And they aren't plastic coated. Polymer is stronger than plastic but not as strong as steel.
Polymer is....(courtesy of wikipedia)
A polymer is a large molecule (macromolecule) composed of repeating structural units typically connected by covalent chemical bonds. While polymer in popular usage suggests plastic, the term actually refers to a large class of natural and synthetic materials with a variety of properties.
Well-known examples of polymers include plastics and proteins. A simple example is polypropylene, whose repeating unit structure is shown at the right. However, polymers are not just limited to having predominantly carbon backbones, elements such as silicon form familiar materials such as silicones, examples being silly putty and waterproof plumbing sealant. The backbone of DNA is in fact based on a phosphodiester bond.
Natural polymer materials such as shellac and amber have been in use for centuries. Biopolymers such as proteins and nucleic acids play crucial roles in biological processes. A variety of other natural polymers exist, such as cellulose, which is the main constituent of wood and paper.
The list of synthetic polymers includes Bakelite, neoprene, nylon, PVC, polystyrene, polyacrylonitrile, PVB, silicone, and many more.
Polymers are studied in the fields of polymer chemistry, polymer physics, and polymer science.
So summed up... It is stronger than plastic and lighter than steel, but just not as strong as steel. It's not a bad thing for the little parts that you have mentioned but if you plan to put a few thousand rounds through the gun an investment of $30 for a stainless steel guide rod would definitely be worth it.