Ten Pound Monkey sent us these photos from the AUSA Expo of the upcoming SIG MCX PDW. The upper receiver contains a short stroke gas piston system with a forward recoil spring, doing away with the need for a buffer tube and allowing foldable stocks and collapsable wire stocks.
SIG are utilizing carbon fiber for the lower section of the handguard, reducing weight while looking pretty nifty, and also in some capacity for the suppressor. Suppressor tubes made entirely of carbon fiber do exist, even for medium sized calibers like .308 Win., but they are very expensive and have a reduced lifespan. It is more likely that SIG are reinforcing steel, titanium or a lightweight alloy with carbon fiber.