AR-Stoner 6.5 Grendel barrel short chambered.

[NugginFutz]

For me it is another "tool", or reference point if you will. If the 123 SST factory round is intended to have the end of the casing and the cannelure at a certain point to provide a consistant length, then if it in too far then the round must be shorter. That is ok if that is where you want it.

Perhaps, and nothing stops you from referencing it that way. Doing so reduces case volume, and that suggests you take time to evaluate your charges, as pressures will be greater than the published loads for that bullet.

Ultimately, though, the only reason Hornady (or any other bullet manufacturer) puts a cannelure on their bullet is to provide a recess for crimping. These are specifically provided for roll crimps, but are not necessary, if you are only taper crimping.