So would my mandrel need to match my bushing size?
Sort of but also not really? The two are independent of one another and it all is dependent on what you want to do.
The bushing pushes the brass smaller from the outside, the mandrel pushes the brass larger from the inside.
You still have the difference in brass thickness and desired level of neck tension in between a labeled bushing and a mandrel dimension.
Ex, brass is .015 thick.
.015 left side neck thickness +.015 right side neck thickness=.030 total combined amount of neck material thickness.
.030 total thickness +.308 bullet diameter=.338 total external neck diameter of a loaded round.
Now, how much neck tension (interference fit) do you want?
A 30 cal turning arbor will measure .002 smaller than a 30 cal bullet so it is .306 and leaves the inside of the case neck at .306. If we add the brass thickness into this the turning arbor leaves the cases with an external diameter of .336.
And a 30 cal expanding mandrel will measure .001 smaller than a 30 cal bullet so its .001 and leaves the inside of the neck at .307. This will leave the brass with an external diameter of .337.
But those mandrels open the case up from the inside out: how do we make the too-large-fired-case thats measuring .342 outside small enough for these mandrels to work the brass? We size them down.
Traditional dies can work it way more than necessary, choosing a proper bushing can eliminate that necessary over working of the brass. So if we want our mandrel to leave the case at the proper dimension it needs to be opening the case up, if we didnt size the case down the mandrel would just pass through the air freely without contacting the brass. In order for the case to get opened up by the mandrel it needs to first be sized down. I like to chose a bushing thats .002 smaller than the outside resultant goal of the mandrel sizing. Since a turning arbor gives us an external dimension of .336 I would want to make the brass a bit smaller first so I would chose a .334 bushing.
Youll need to alter these dimensions for your caliber and brass thickness.
Or you can skip a mandrel altogether and just choose a bushing for the amount of interference fit you want and be done with it.