James, I have neck sizing dies, I have full length sizing dies, when I use a full length sizing die to neck size I adjust the gap between the top of the shell holder and bottom of the die. "HOW MUCH?" I adjust the gap between the shell holder and die to the amount of the neck I do not want to size. .100" would be a big gap, you indicated you used a gap of 500"+.
The part of the neck that is not sized will aid in centering the case in the chamber. I always ask about the difference between firing full length sized cases and neck sized cases, in the past there are those that feel they have to say something.