Regarding using a mandrel:
The basic concept of course is to size the neck back down enough to hold the next bullet you put in it. If you try and put a bullet in an fired cartridge, it SHOULD just slide in finger tight at the hardest. If it doesn't, well, you may all sorts of other issues.
You get the neck sized down by forcing it into a smaller die (a hole smaller than it is). This is the basic concept, step 1
Standard dies force the neck WAY... down and then open it back up when you pull the expander button back through the neck. Standard dies are made to deal with all the extremes. Thick neck, thin necks, uneven necks. A standard die set sizes the heck out of the brass and relies on the expander to open it back up. It compacts and stretches the brass a lot, and the brass gets hard when its "worked". So case life is impacted.
Some guys have dies that have bushings that neck/reduce/size the neck according to their wishes. Works the brass less. Done perfectly, with very uniform brass, or neck turned brass, you can then just shove a bullet in the neck and you are good to go.
But some of us are not shooting a gun that has perfect Lapua brass, and/or we are not interested in neck turning every case. So just sizing from the outside (bushing), is not quite right. Imagine then that the "thicker parts" of the neck are now "on the inside". It will grip a bullet, but its inconsistent from round to round.
Enter, the mandrel. You can buy them in .001" increments, and run them into the case after its been sized from the outside. A mandrel pushes the irregularities of the neck back out to the outside, thus improving neck tension consistency. Why is a mandrel better than a sizing ball or expander? They both perform the same basic function.
Well, for first, the makers of mandrels are more exact in their dimensions. And the normal expander balls are more variable.
But also: Pulling the expander back out of a case tends to lengthen/elongate the case. This can cause problems with chambering at times. The mandrel will do the opposite, it pushes the shoulder BACK a touch. So you have less chambering issues.
In the end, there is no one proven solution. But there a lot of very good shooters using mandrels as the last step.
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