I was measuring some dimensions on a RCBS 15501 FL sizing die in order see what was happening to the brass. The neck bore is 0.307 measured with a gauge pin set. LC brass resized with this die has a neck ID of 0.278. The expander ball measures 0.3065. You can see how much a stock die works the brass as opposed to a neck bushing die. I use a 0.334 bushing with the same brass so the RCBS die is 0.026 smaller at the neck.