If you have an A2 style front sight base on your AR, but no rail coverage between it and the birdcage, I’ve found the product for you: the KeyMod Forward Rail (KFR) from Ergo Grips.
This rail system bolts down to the front sight base and adds a few extra inches of accessory rail real estate farther out the gun. While my initial reaction is that this is a perfect Mall Ninja® add-on, I can see where it might have some genuine purpose.
Consider the standard AR-15 with its round hand guards and FSB. If you wanted to add a light to the gun this KeyMod rail would allow you to do so with relatively little fuss. I’m sure you can think of other possible uses for this additional rail space.
I like Ergo products, but if I was adding rail space to a round hand guard AR, I’d just install one of the Magpul hand guards. Those hand guards are a direct replacement and take very little time to pop into place. Of course, the Magpul products to not extend the rail space beyond the FSB as the KFR does.
Ergo Grips makes the KFR from 6061-T6 aluminum and gives it a black anodized finish. The mini quad rail has a suggested retail price of $138.50.