i use .308win cases necked down to 7mm08., your cases will be short in the neck, but not by much and they are fine. One thing you might find is that, you will have trouble chambering the newly loaded round. this was the case with me. it was not neck thicknes it was shoulder issues. I ended up making a slim shim out of a piece of coke can and put it in the shell holder under the case. also when you start on load work, start low and work up. when you have fired a round take the un resized empty case and try to fit a new bullet in. It should get in no problem. if there is no way it is going in you might need to turn the necks or your pressures might be high.
steve, uk
ps you can also size up .243win