There may very well be a small piece of dirt sitting in the neck area. Upon firing, the case is expanding and denting, just like sizing a case in a dirty die or one with to much lube on the case. Another possibility is, you may be leaving some solvent or oil in the chamber and it is denting the neck. If you are not using a bore guild, try and run an over sized patch in the chamber area to get any solvent that may be left there out.