Tennessee Arms Company have published a video showing a prototype clear polycarbonate AR-15 lower receiver.
This is just a prototype and not yet available for purchase. Prototype transparent lower receivers have been made before, but I cannot think of any that have made it to the market. Transparent polymers are usually not as tough as equivalent opaque polymers. This is because a low degree of crystallinity is required for light to pass through a polymer, rather than be scattered, but a higher degree of crystallinity is associated for toughness (impact-resistance and ability to bend back into shape). So you can generally either have a transparent but brittle polymer or a tough but opaque polymer, but not both.