A few questions:
What brand of die? For my curiosity's sake.
Did you lube the outside of the case? Did you lube the inside of the neck?
To answer your question:
-If the case you're reloading is new, you need to use the full length resizer.
-If the case you're reloading has been fired in any chamber other than the one you're planning to fire it in, you need to full length resize.
-If you fired the case in one rifle and plan on using it in another rifle, you need to full length resize.
-If you plan to use the case in a semi-auto, you need to full-length resize.
-If you fired the case in one rifle and plan to use it in the same rifle, you CAN use the neck sizer (or full length size).
The point of the neck sizer is that when a case is fired in one chamber, it assumes the shape of that chamber, and therefore does not need to be full-length resized. It needs neck-sizing because there needs to be neck tensioning to hold the bullet.
All chambers are different, even those of the same caliber, so you can't necksize a .308 you used in Rifle A and expect it to work in Rifle B (it MIGHT work if you're lucky).
Why use a neck sizer in the first place? Convenience. Using a neck sizer requires no lubrication, and requires less force than full length resize. It is therefore a lot easier to run 50 cases through a neck sizer than it is to run 50 cases through a FL resizer.
A tip: When you neck size, check to make sure the neck is small enough. A few days ago, I neck sized 50 .223 cases and when I went to seat bullets, I discovered that I did not apply enough force to the sizer, and I got no neck tension. Had to go back and neck size them again.
Another tip: Don't apply too much force. I broke my neck sizing die by putting too much pressure on it (it's a Lee).
