There's a roll pin that holds it in place, they don't put it in the manual because it's not recommended to be done at the user level. I would not recommend doing it either, especially since there's easier and better methods to achieving your goal.
1. Buy the Hornady OAL tool and modified case. Easy button, super accurate/easy method. If you load ammo for a precision rifle and DON'T have this tool then you are doing life wrong.
2. You have an AXMC... remove barrel entirely and measure using a fired case and the cleaning rod method.
3. Do the method you're looking to do, but with the barrel out, which is nowhere near as accurate as the two methods above.