For the best brass, neck up 243 Lapua I do this but have to turn the necks to prevent doughnuts . I also neck down Winchester 7-08 brass and it shoots very well. No neck turning.
I've never necked anything up, just down. Story is when necking down, the thinner neck material is "pushed back into the shoulder." When necking up, the thicker shoulder material is pulled into the neck creating a doughnut.
When Remington first came out with a 700 in 260 my hunting buddy got one. You could not even find any ammo much less any cases to reload. There was not any loading data out either. We tried some 308 cases necked down but the neck was quite a bit short. We then necked up some new Remington nickle 243 cases that I had laying around. They really did the trick. These were the most accurate cases he ever found for this rifle. We had no data so we just started with a 6.5X55 Swede start load and worked up with H4350. We found that 47 grs was about all you could get into a case and seat a bullet and that is were this rifle really shot with 120 gr Sierra SP bullets and 45 grs with 140 Sierra SP's. We did not have a chrno and the cases and primer showed no over pressure signs. That rifle had a 24 inch barrel and I would like to have know what FPS we were getting. We used this rifle to shoot deer on a farm we control hunted. My hunting buddy let me shoot a couple of deer with it but he killed 52 deer that year with it using both loads and out of all the deer we shot with this rifle only 1 took a step after being hit. This deer took 3 jumps before it piled up. He killed quite a few deer between 300 and 350 yards with it. My hunting buddy is one that has to try out everything new that comes out so by the next year he had sold that rifle and bought something else. I which I would have known that he wanted to sale it because it was a real shooter.