Mike is right, and I should have mentioned that you can partial neck size or even partial size body and neck with a full length sizing die. But a neck sizing die is just that.
I've been reloading my 6 m/m Rem. using the same old C&H .244 Remington Neck sizing dies my dad used. Most here know they are one and the same case, and only the factory rifling twist was different. I never owned a 6 m/m full length sizing die, and never needed one. I've reloaded my brass to the point where I started to see necks cracking, and discarded it, but it still never needed the body sized. If I ever bought some used brass that had been fired in a different rifle, it's entirely possible it wouldn't chamber well in my rifle after neck sizing only.
The one complete head separation I had with a .22-250 that blinded me for roughly half an hour happened after I reloaded some very used brass that had been fired in a different rifle which had less headspace. I was in my 20's and had bought a .22-250 on a VZ 24 Mauser action. A friend gave me his old .22-250 brass and a set of dies. I had set up the sizing dies to just kiss the shoulder of that old brass, but didn't know it was still short for the chamber of my new gun. It showed bad primer flattening on the first two shots which alarmed me... but not enough to quit and find out why. I decided to fire one more, and all hell broke loose, and it felt like Mike Tyson punched me in both eyes. In an instant, I was totally blind.
I learned in a big hurry that correct sizing and properly setting up dies is super critical. Neither me or the rifle suffered permanent damage, but if you'd seen the brass and burned powder particles embedded around my eyes, you'd know I was extremely lucky. The shock of the blast must have shut down my optic nerves. I also later learned that Paul Mauser had redesigned the 98 Mauser bolt shroud to deflect gasses around a shooters eyes in the event of a rupture. He did good!
