Re: new 6mm?
The 6mm Remington is indeed about all the powder the 24 bore will burn contrary to popular belief. It doesn't matter whether you go a 30" barrel a 24" barrel, the results end up being very close to each other. The .240 Weatherby case is smaller in volume than the 30-06 case used in the 6mm-06, and yet with it's higher chamber pressure shows maybe 100fps faster velocity than the generic 6mm with the help a lot of free bore. Add that same free bore into a max load from the 6mm, and you'll see similar velocity.
The 6mm-06 has a very slight more case volume than the 6mm/284, and that round is famous for making barrel manufacturers rich. I shoot the .250 Savage case necked down to 6mm and improved. It will do 3000 fps with a little over 44 grains of H1000 and a 105 grain Amax. That's close to 150fps less than the 6mm with the same bullet, but also using about five grains less powder. But on the otherhand the better case design in the 6mm will probably give you a 50% increase in barrel life. Yet the 6mm improved case will give even better barrel life yet will be slightly overbore (not enough to be alarmed about). The .243 case is well known as a poor design, and is rather hard on barrels. Ackley did an improved version, and maybe increased the throat life 33%. Yet still way short of a standard 6mm Remington. Then we get into the 6BR that somebody mentioned here. That barrel will last 50% longer than the best of the rest with ease! It'll push that 105 grain bullet to 2800fps and some folks are getting 2900 fps; yet uses about 2/3rds the powder.
Of course if your one of the guys that only shoots a hundred fifty rounds a year, then any of these rounds will do you just fine. The only serious difference is the cost of powder, and that still is not much. But if your thinking about a barrel life of 1500 / 2200 rounds, then you better look for a non over bore cartridge