I don't have any drawings before me but I suspect by the time you adjusted a set of .243 dies down low enough to do much neck sizing, you would also be resizing part of the body of the case and reshaping the taper. Any bottle necked cartridge it's best to have the EXACT dies for the cartridge. JMHO If I went to the trouble to rechamber a rifle, I'd spring for the correct dies as well.