I understand that the brass is fire formed to the chamber and it will be a perfect fit, therefor may be snug.
What are you using to neck size with? If you are using a FL sizing die and just not taking the case all the way into the die to neck size, realize that you are probably moving the shoulder forward. If this is happening, you are reducing your headspace and thus the added force to chamber.
If you don't understand why the shoulder moves forward, I would be glad to explain.