Which shoulder bump/headspace gauges (other than SAC) index off the entire shoulder rather than a datum line?

[secondofangle2]

As the (long) title says. SAC has a shoulder machined into their comparators, as opposed to the Hornady I have which just uses a datum line/point on a particular point on the shoulder (e.g., for 308, datum line is where the shoulder diameter is 0.4", I just learned). What other options are there for this way of doing it like SAC does? Whidden? RCBS precision mic? LE Wilson? Redding?

Another question is, is this the best way to do it, the way SAC does? I have the intuition that it is a superior method, but no way to know or even support this intuition.

 

[FLIGHT762]

Sinclair inserts have a shoulder. It is a comparator and does not give SAAMI numbers. On a 1.630" GO gauge, my Sinclair 308 Win. insert measures @ 1.561".

If you are looking for a SAAMI number, the RCBS precision mic and Whidden gauge are zeroed @ 1.630".

what is the measurement off of the SAC 308 insert on a 1.630" GO gauge?

 

[CK1.0]

I just use the same datum SAAMI specifies with the Hornady comparator.

For 6GT I use the .375 bushing (as shown): https://saami.org/wp-content/uploads/2022/01/Public-Introduction-6mm-GT-2022-01-28.pdf

For 6CM I use the .400 bushing (as shown): https://saami.org/wp-content/uploads/2018/04/6mm-Creedmoor.pdf