All jokes aside, your first point is well taken and that's why I'm here. so in your chart above, those are all well understood except "Exp"... is that expected?
That is "expanded". An easy general rule of thumb is that you want your finished sized brass to be~ .002 smaller than a loaded round, aka you want .002 of interference fit between the brass and the bullet. The amount of interference fit is one variable you can use for load tuning but thats way into the benchrest point of diminishing returns. For you we just need to make functional quality ammo before we dive into the deep end.
My sizing die on that example is a standard non full length sizing die with no bushing, so I cant control how much the neck is sized. With the expander ball removed the die body itself sizes my outside neck diameter down to .265 which would result in .006 of interference fit, if I were to seat a bullet directly into that it would function fine but it would be a bit more interference fit than I would like and could possibly deform the bullet.
Because of that your sizing die comes equipped with a steel expander ball. When the expander comes back up through the now sized down neck it will force the neck to open up. The expanders are generally that .002 below bullet diameter goal we aim for so that all brass will have the same internal diameter (provided its getting sized down enough in the first place which is your issue it seems).
SO lets make some examples for your situation:
You have a bushing that is .335 on its inside diameter, that means it will size the brass necks outside diameter down to .335 minus any material spring back (spring back is always towards the direction it was sized from)
Your bullet is .308"
So if we subtract .308 from your .335 bushing diameter we get .027 of space in between the two. That is the space around the bullet that would be filled with brass, aka your case neck. Because the diameter is all the way across the entire round that means we have both sides of the neck accounted for here. .027 divided by 2 equals .0135 which is how thick the case necks would have to be to get any sizing at all.
If your case necks are only .012 thick, like many are, then we get .308+.012+.012=.332 That is smaller than your bushing is so even though the bushing will size its outside diameter down some from fired it wont be far enough for the thinner case necks inside diameter to hold the bullet like it should.
So, we want the bushing to be
at least .002 smaller than your loaded round diameter.
I like to go .004 below the loaded round diameter and then get let the expander open it up to the final .002" below loaded round diameter. I believe that if there are inconsistencies in the case neck thickness then sizing down pushes the thicker parts more to the inside and then expanding back open pushes the thicker parts back to the outside presenting a nice circle on the inside of the neck to interface with the bullet.
If you have good brass that doesnt have any inconsistencies in the thickness then the benefit to the extra sizing and expansion step are moot as there are no inconsistencies to consider.
Some people believe that the bullet will push all of those inconsistencies to the outside itself and that a .002 below loaded neck diameter bushing alone with no expansion is all thats needed. I cant offer any empirical evidence to the contrary but I still like the logic of the above.
(The reason I said my die wasnt a good match for my chamber isnt from the neck, its due to the base of the case where .4695 stays the same .4695, its not getting sized any so its the same size as the chamber and makes for tough operation because Im having to muscle it to fit instead of being sized properly and fitting freely. But you can see I take all of the measurements and I can see where any conflicts are happening.)
