Might take a look at my post on reforming .243 brass to 6.5 Creedmoor. Yes....308 is a parent case,but youi will damage more of it than the .243 cases because ot the sharper shoulder slope on the .308. My method was a single sizing in the 6.5 die and cutting the neck which was extended about .125. Test fired successfully in a bolt action rifle. The extractor groove of the .243 cases might have to have the edge beveled for better purchase when chambering,as it tends to bind a bit,,,but still chambers OK. A cartridge head space gauge is critical for proper sizing prior to the final cut off.