1. Micrometer resizing dies. Do these exist? If not, why not?  A lot of us have multiple guns that shoot the same cartridge, but often the amount of shoulder setback will differ. Short of buying separate resizing dies for each gun, wouldn't a micrometer type die like what seaters use help?

2. Can you use one seater that's specific to a caliber for all cartridges? I.e. the same seater for 308 Win and 300 BO? Obviously adjustments might need to be made... But the bullet is the same for both cartridges.

These might be stupid questions but I thought I would ask.

What questions do y'all have about reloading that you've been afraid to ask?

1.  Not that I know of.
2.  No.

RCBS does make a "Precision micrometer headspace and Bullet seating Depth Guide".

I'm not as familiar as I should be with it, it's one of the things that's in my die box but I haven't used it enough to be an authority on it's operation.

1: Not sure but being a career machinist and basically reloading for almost as long I'd say the word impractical comes to mind. They do make tools to accurately measure this and it's supposed to be done each time.

I say impractical because one simple fact. All brass is not the same. Some has more spring back or simply resists sizing more than others so you need to measure and possibly adjust every time.

2: Again not practical. Seating dies are made to guide the case into the center of the die so that the bullet has the best chance of starting into the case neck centered.

To do what you suggest would mean a very shallow die with a large bore that would offer no lateral support for most cartridges.

Motor

Get a cartridge headspace gauge and these. Best you'll get. By best I mean only option, that I know of anyway...

https://www.midwayusa.com/product/525874/redding-competition-shellholder-set-1-308-winchester-30-06-springfield-45-acp

https://www.midwayusa.com/product/359546/redding-competition-shellholder-set-10-17-remington-204-ruger-223-remington

You could use the Redding competition shell holder kit. Might take some tinkering though.

https://www.whiddengunworks.com/click-adjustable-sizer-die/

I'd rather go with Redding Comp shellholders.

Solution to your problem = Redding Competition Shell Holder Set >>> LINK.

+1

Me thinks he meant Redding Competition Shell holder sets.
You can look at using Skips die shims available for Sinclair
You can look at the Whidden die