Dual flywheels in vertical layout (similar to the Nerf Barricade), but in a much larger scale.
The new blaster designs from 3rd-party brands are starting to look much better nowadays, the Quick Fire 48 even has a removable shoulder stock... which has a rather uncanny resemblance to the Nerf Raider shoulder stock. :)
Photos of the box:
Photos: Toys R Us USA
Product Link at Toys R Us USA website, Click Here.