I have a set of Hornady Match Grade dies for .308 that I know longer use. If I remove the pin/innards from the FL sizer, will be still bump the shoulders without effecting the neck? (I have a collet die for neck sizing).
I speak from experience having tried the same thing. Your die will still squeeze the neck down and, at least in my case, the mandrel from the Lee collet neck die wouldn’t fit into the neck. Unfortunately, it doesn’t function the same as a body die.
Either cut the neck completely off or buy a Redding Body die. A body die and Lee Collet neck die will produce extremely straight ammo.
This is how I size my 6AI brass, my bullet run out is less than .0015" av .
Removing the expander ball from a FL sizing die will result in the neck being sized down smaller than with the expander ball. The opposite of what you're trying to accomplish.