Flat ground hardened machine washers are available. Try E-bay.
The question "How do you measure it?" is interesting.The definition of concentric is sharing the same center point or axis.
How do you establish the center point or axis of a loaded cartridge?
You can't put it between centers to establish an axis.
Vee blocks,or rollers? Is roundness considered? Concentricity or coaxiality?
Is straightness considered? Cartridge head and neck? Neck wall thickness? Neck straightness? Or work off the case head and the bullet?
Whatever works for you,but if you get real technical,whatever guage you have might give you a useful number...OK,fine.Close enough!
But to give you true measurements,you might need a co-ordinate measuring machine,or you might need to establish Datums. Theoretical circles or cylinders, and those might be represented by a precision collet,or it could be a a center with a throat or bore dia to engage the bullet ogive.
No doubt Sinclair or Hornady,etc will sell you very nice tools with indicators and they are useful.
But its unlikely you can measure "Concentricity" per ASME Y 14.5 Geometric Dimensioning and Tolerancing unless ,for example a datum diameter and location is established at the case head and perhaps another datum diameter to represent rifling engagement to the ogive,then a T.I.R. spec perhaps at the shoulder datum diameter or .100 behind the shoulder....
The minimum/maximum material condition come into play as does the chamber dimensions ...cartridge/ chamber clearance...
IF what you are trying to control is the bullet being coaxial to the bore at ignition. Or obturation??
Enjoy what you are doing,make good,straigt ammo...Happy shooting!
Just realize the statement "My ammo is concentric to .002 " is a pretty complex statement...and subject to review.