The service rifle in NV was I though a fantastic piece of world design. Its a weird mix of AR-15 and AR-10 in fallout fashion but the wood furniture said one of two things.
The pre war world used wood stocks either by design or necessity as the resource wars dragged on. I'm thinking more the later since the marksman carbine was a similar design but used a composite stock like we would see in our world. That gun also seemed like a specialist weapon, they wouldn't have needed to produce them in mass like the service carbine so the use of composites or plastic is justified in a world running out of oil. That or the gun runners are building the service rifle with wooden stocks because they don't have the technology to produce composite stocks. I don't imagine melting down scrap aluminum and milling it to spec to be relatively difficult vs layering or finding suitable plastics to build a composite stock.
I am probably thinking about it too deeply but this was always a really cool design in New Vegas for me.