Re: Shims for dies to aid in headspace
"The Redding Competition shell holders will allow you to size down a little more on the brass."
Not so. The Redding "Comp" shell holders are made thicker to reduce and control the possile amount of shoulder set back.
No one needs a set for "every" cartridge he loads for, only need one set for each case head size. Even then, if they help at all they're only useful for those cases we may want to squeeze out the last few thou of possible accuracy, they sure won't make a massive accuracy difference.
Any case body contact with a FL die during use changes the case body and is NOT "neck" sizing. Some case/FL die configurations will permit neck sizing by backing the die up a bit but it's not common, especially if the loader wishes to size the full length of the necks; that's why neck dies are made. If all it took was backing FL dies out a few thou no one would buy neck dies. Not that neck sizing has any real value in factory sporting rifles anyway.