The 240 Wby is smaller in dia than the 243. You would need to go 6-284 to clean up the chamber. But, if it has to be factory he is pretty much out of luck.

This would be for a buddie of mine. And factory ammo would be a important factor. Thanks for the reply Maddog

6/284 would be a better choice i think..........
woofer

Dan, this is fairly simple. The shoulder of the .243 Winchester is larger than the diameter of the case on the .240 Weatherby just ahead of the belt, so to clean the chamber up you have to go just behind the Winchester shoulder. In other words, the only place on a .240 Weatherby where it is equal to or greater than the diameter of the .243 Win shoulder is at the belt of the Weatherby case. Ain't gonna work.

Dan Belisle,

If you check out the dimensions of the .243 you will find that the chamber will not clean up with a few threads and taking some off the shoulder. The diameter of the case on a .240 Weatherby is .453 just ahead of the belt and the shoulder is .432. On the .243 Win, the diameter of the case just ahead of the groove is .470 and the diameter of the shoulder is .455. The shoulder distance on the win is 1.553 and the Weatherby is 1.992. It will definitely NOT clean up.

"And factory ammo would be a important factor."

Then I would leave it alone. If your friend wants to use factory ammo I would assume he is interested in big game hunting not, varmints. If so then the extra speed of the Weatehrby is unnecessary and undesirable, IMHO. Too much velocity will make for critical bullet selection. Normal non premium bullets will explode on impact. This is not a good thing. You will make a bloodshot mess. I have no use for a big game cartridge that is faster than 3000 fps. Second, the trajaectory is pleanty flat in the 243 Win. You gain no practical extension in the big game range with the Weatherby. When you get way out there you still need to laze the range and hold high. It is a matter of how much. Also, 99.99% of guys can't hit reliability way out there and should get closer. Your friend would benefit more from shooting 10 boxes of factory ammo then spending the same money on converting his rifle to an obscure caliber. .........I think. : )

Dan,

Just curious. My books show the 243 to be .455" at the shoulder while the 240 is .453 just ahead of the belt. The 243 is also .470 at the base. How would a 240Wby clean up the chamber?