I have thought about necking that case on up to 30. GEZZZZZZZ I think that would be a shooter. Velocity should be in the 308 range or better I would think. Good luck with your present project. I am a big fan of the 260, but I know I can't get 2900 fps accurate velocity out of a 260, you have my interest with that project. Initially I am going to just neck up the 243 WSSM to 6.5 without further modifications to see what it will do. The 35 degree shoulder maybe next depending on what happens with the first experiment.
I have a Remington short action that I had opened up to build a 300 WSM
on and I am thinking it would be very suitable. The bolt face is perfect and it has been blue printed. I am thinking it would work through the magaizine even when throated long making it a good long range varmint cartridge if that was on the agenda. I would bet the WSSM aciton would not feed a long throated 6.5mm. The 300 WSM barrel that is on this action now is nearing its last legs with 1300+ rounds on it. So I am going to build it on that action. If it shows promise with that barrel I had given to me, I will order a barrel and chamber a fresh barrel on it. IF I didn't have that action with the proper bolt face, there is no doubt in my mind I would wait for Wincherster, not Browning to come out with the WSSM sized action.
When the 243 WSSM ammo comes out I am going to buy a box of it pull the bullets and neck up to 6.5, do the rest of the case prep work and seat a bullet to .100" more than full neck tension with a 140 Berger and have the reamer ground so the bullet just touches the lands at that lenght. I am thinking seat it just a little deeper than full neck tension so I have a some room to chase the lands and still have full neck tension.
Buddy, send me an email the last one I sent to you got returned as undeliverable. This is Gary from southwest Indiana, maybe you don't recognize me as RiverRat.