Whelenman,
The 6mm comes close to the .240 only if you load it pretty darn hot.
With a 100-grain bullet seated to standard factory overall length, the .243's water capacity is around 50 grains, the 6mm's 52, and the .240's 58.
Of course, capacity will vary a little bit with brand of brass in the .243 and 6mm, but not enough to change the ballistics substantially. They are far more similar to each other than to the .240.
