Melvin Forbes at one time had a short necked version of the 6AI that still had a longer neck than the 243. I assume you could form them from .257 brass in one pass then fire form so not much more effort than the 6AI. It would come closer to splitting the difference between the 6mm and 6/284.

Sort of a 240 short Gibbs.

"When you disarm the people, you commence to offend them and show that you distrust them either through cowardice or lack of confidence, and both of these opinions generate hatred." Niccolo Machiavelli