I have only formed a little 308-243 0ver the years. My one 243 is a tight necked chamber with .267 neck so all brass for it must be turned. Several things need to be watched when doing this, 1) neck wall concentricity. resizing can cause uneven wall thickness. 2) neck wall thickness can thicken sizing down making rounds tight to chamber and no release room when fired. Sizing down thickens the neck sizing up thins the neck. 3) Depending on shoulder angles neck dias and body tapers of the 2 cases brass can shorten or lengthen.
There is a set amount of brass in the parent case resizing this the brass flows to make what is formed with that brass. cases can lengthen since the brass can only move forward, sides and base are fixed sizes.
Necks can be addressed by reaming or turning. reaming thins the neck to a safe thickness but dosnt correct the wall thickness variatiions. Neck turning with a special tool and mandrel corrects the wall thickness and wall thickness variations to a very close wall thickness. Neck turning needs to be done carefully to avoid the dreaded doughnut at the shoulder neck junction. Some of my best wildcat cases were both neck reamed and turned. Necks were reamed in a die by hand .002 small sized on the turned mandrill and neck turned to size. This gave a neck wall thickness needed and a variation of .0002 on thickness.