Originally Posted by lloydsmale
ive got both a 240 and a 6mm rem. Loaded up to the cases show pressure signs and then backed off a full grain the 240 will beat the 6mm by an easy 200fps and with some bullets 250 fps. About the differnce between a 280 and a 7mag or a 06 and a 300 mag. If we worried about a cartridge thats over bore non of us would own a single magnum rifle or even some of the standard calibers like the 2506. The 6mm is a great round but its sure not the end of performance using a .243 bullet.
240 WBY cases are about as common as my ex wife's smile in court! The 240 will drive a 105 grain Amax to about 3250 fps max. The standard 6mm will drive the same bullet to about 3100 fps, but with about five grains less powder. Yet the 6mmAI will add another 150fps to the 3100fps and still be safe. But the other thing is that the barrel life is good. Looking thru three reloading manuals I see less than 100fps difference, but most often I find the 6mm on the anemic side. It's a true 65K psi round, and you gotta get that data. At 63K psi in the 6mm there's just not a serious lot of difference. Speer says it will do 3145 fps with a 105 grain bullet, and that's about right. I don't know how they managed that from a 22" barrel but they did. A 24" or 26" barrel will easily do that.
Still the real issue here is hitting something at five hundred yards. The 6mm will, and it's a maybe or maybe not with the 240 and all it's free bore