I read that during the development of this round, Roy Weatherby had a die made, probably a lot like this one, to size '06 cases to create the belt. He went with the '06 head size for obvious reasons and the final SAAMI .240 case dimensions reflect just how much set back the case body needed to create a belt that looked like the rest of the Wby. family.
The .257 Wby, with the powders available at that time, was considered somewhat "overbore", what ever that term actually means. It appears to me that with his new 6mm round, he wanted a smaller case and the '06 was the logical answer.
IMHO, this was purely a cosmetic ploy so that the new round would look like the rest of the Wby. family and not a "redheaded stepchild". For Roy, belts were his trademark, whether needed or not! And, it's common knowledge that one can't use the word, "magnum" without the round having a belt...
The .240 Wby. is a squeezed down 6mm-06 with slightly less powder capacity, thanks to the "squeezing". Other than proprietary marketing, I see so purpose served by the belt, other than to sell super expensive brass. Ole Roy wasn't a fool...
IMHO, of course.
DF
