If you want a 6mm/.243 then get a .243 WSSM upper. It puts the .243 Winchester in the obsolete category. What, don't want a .243 WSSM? Then how about a .257 WSSM? Or maybe a .300 WSSM or .358 WSSM? I have been living with the .243 WSSM for quite some time in my home made varmint rifles, and can declare it superior in accuracy to all the others. After all, it has the short fat case shape lauded in the 6mm PPC, superior capacity and efficiency with medium burn rate powders such as Hunter and the 4350's. If you like something with a 1:8" twist that can obliterate ground squirrels like a Klingon disruptor and take everything else with Barnes TTSX bullets, while shooting sub-MOA groups, then what are you waiting for?