Bushing die help

Once you squeeze the neck from the outside that pushes any variance to the ID of the case. If you run a mandrel it makes the ID of the case properly round and any of the inconsistencies will be pushed outside and not have affect on the projectile.

To select a bushing I take a loaded round and measure the OD and -.003" then I run a mandrel .002" under bullet diameter.
Now you see, that actually makes sense to me, compared to the OP basically using a Mandel like a standard expander ball after over sizing the neck.

Now in theory with a turned neck there shouldn't be any inconsistencies in or out but if you are using unturned brass, keeping the variance on the outside is good. Sort of the same idea as Lee collet dies and why they are so effective.
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Bushing die will reduce over working the brass. The mandrel also does correct the neck i.d. inconsistencies. A regular full length die excessively works the brass in the neck. You just need to figure out the right combination. Brass thickness comes into the arithmetic so measure the neck thickness.
I do not understand the point of using bushing dies and an expander mandrel. I use the correct bushing to give me 3-4 neck tension on hunting rounds, end of story. I see no point in over squeezing the brass and then opening it back up with the mandrel. Sort of defeats the purpose of a bushing die.

What you are missing is that bushing dies do not size the case neck straight, the Mandrel acts to uniform and straightens the case necks. 1000 yd benchrest shooters learned this trick a long time ago. Serious accuracy freeks will have sets of mandrels graduated in .0005 increments.

Obviously having an instrument to measure runout is critical when you get to this reloading stage.