I found this info on the guns and ammo website.
"Winchester's newest entry in the 6mm field is the .243 Winchester Super Short Magnum (WSSM). This case is nearly identical to the .223 WSSM but necked up to 6mm. It's much shorter than the standard .243. The performance numbers are interesting. While the two cases have almost identical volume, the WSSM produces factory velocities about 200 fps faster than the .243 Winchester. That sounds like a very strong argument for short and fat until you look at the factory pressures. The .243 is held to a 60,000-psi pressure level while the .243 WSSM pushes 65,000 psi."
So it would be flatter shooting but your barrel wouldn't last near as long. I looked at this round before getting my .243. I still am intreged by the little fat round though.