I've got plastic guns, but I ain't gonna buy no magnum revolver TOP BREAK with a plastic anything.
The top break won't handle the pressures even if it's solid steel.
Now, then, has anyone tried carbon fiber? It's used to make wheels and even disc brake rotors on racing motorcycles. Ultra light, ultra strong. I'm not too sure about shear strength and other stresses that would involve a firearm, though. I'd have to let the engineers figure that one out. But, it'd have to be some kinda strong to build a magnum top break revolver out of it that would hold up.
A top break is a cool idea, they're just inherently a weak design.