The AR-57 looks really interesting. It’s an upper receiver that allows one to shoot 5.7x28mm ammo (the same as the FN Five-seveN and P90) from an AR-15 platform.
It takes standard P90 magazines on the top of the handguard. From the pictures on their site, the magazines don’t come up any higher than the optics rail.
According to the discussion at AR15.com, it shoots well. It also drops spent cases out of the magwell, which should make for easy gathering of one’s brass, as 5.7mm is crazy expensive.
Hat tip to Uncle, along with his suggestion to use a Crown Royal bag with a rubber band to catch the brass.