What a sizing die *actually* does
A visual walkthrough of full-length sizing, neck sizing, shoulder bump, expander balls, and mandrels.
I am more of a visual learner, and sizing brass did not really click until I could watch the case move through the die. I made enough bench errors - too much shoulder bump, not enough, expander-ball drag, mandrel confusion - that this little visual tool finally made the process make sense.
Inside the sizing die
Scrub the cycle, change the die setup, and watch what the steel is actually touching.
The sized case starts with clearance. There is room for the bolt to close, and pressure has not expanded the brass into the chamber yet.
Headspace is the trap
On a bottleneck case, the bolt cares about the base-to-shoulder datum. Too little bump gets sticky. Too much works the web harder next firing.
Body and shoulder
A full-length die squeezes the body, moves the shoulder, and closes the neck. Die depth decides how much clearance you put back.
Same chamber only
A neck die leaves the body and shoulder in the fired shape. It can feel elegant until the bolt gets heavy or the brass needs to run elsewhere.
Ball or mandrel
An expander ball pulls back through the neck on withdrawal. A mandrel sets the inside diameter in a second pass, with less shoulder tug.