[Alpo]

No, neck sizing is squeezing the neck down.

Your bullet is .308. The diameter of the bullet-section of the chamber of your rifle is gonna be something like .350. Let's say your brass is .015 thick. That means you got a 308 bullet with 015 on each side of it, making a total of 338 diameter. There must be some room for the brass to expand to release the bullet. so let's say that the bullet section of your chamber is 350.

You fire the gun, the case expands until it hits the chamber walls and the released bullet goes downrange. Now your fired case has a neck OD of .350, and since the walls are .015, the ID is .320. You put a 308 bullet in that 320 neck and it will fall back out.

So you need to squeeze the neck down, so that the ID is about 306, so when you shove that 308 bullet in there the neck holds it tightly.

The case body, which started at .441, has expanded to .460. The shoulder, which started at 1.948 is now at 1.953. It is now "fire-formed" to fit the chamber of THAT gun. When you full-length resize a case, it squeezes the body back down to .441 and pushes the shoulder back to 1.948, along with squeezing the neck do to an ID of .306.

When you neck-size, it does nothing to the body or the shoulder of the case.
All it does is size the neck back down to that 306 ID.

A cartridge that is loaded into a case that has been full-length resized will fit into any 30/06 rifle.

A cartridge at has been neck-sized will only fit easily into the gun it was fire-formed in.