Take a look at the 6CM.

It is a long 6BR or a 243 Ackley with a 30' shoulder.

The nice thing is you can use real slow powders like H1000 to save the barrels throat, the 30' shoulder still feeds through mags and the 30' shoulder keeps brass growth down to a dull roar compared to a normal 243.

And 115 gn bullets are cheap and dont recoil much when compared to the 6.5 142s.