Ah, the mystic of shell holders.
It's not difficult to measure shell holders to determine variation from the industry standard of 0.125.
One example I have is 2 RCBS #3 shell holders. One very old, one newish. They both measure different so would give different results. The old #3 has been stored away.
I used to use the Redding Comp set for a while but as they only covered the 308 family, and I didn't wish to buy several sets I instead went to Skip Shims. They come in a set of about 8 IIRC so you can build combinations. They go on your die up against the die locking nut. Adjustments can be made in 1 thou steps.
I didn't like the Redding Comp in the end as I think their design should have the middle shell holder at 0.125 and the the others either side getting smaller and bigger.
I measured the Redding Comp ones at 3 points. They are also not true 2 thou steps, close though.
Mine are quite old so more recent ones may be better.
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