Kit-up has
three new posts talking about a new concept SAW style weapon that fires a telescopic lightweight 5.56 round (essentially a polymer case with the propellant surrounding the projectile.) Removing the brass from the equation creates a pretty decent reduction in the overall weight of the ammo.
They're also talking about a traditional carbine setup using the system (possibly a modified M4.) The shape and size of the round allow it to use a 40 round straight magazine (not curved like the standard AR/M16 mag) that is not much bigger than the current M16 mag. The operating system also lets them squeeze a few more inches of barrel into the same overall length package, which is never a bad thing.
From what I understand, the tech has been around since the '70s, but it sounds like the Army is hoping to actually start seriously looking into it, with field tests slatted for next summer.