AR-Stoner 6.5 Grendel barrel short chambered.

[woohoo]

Another way of putting this, is that SAAMI certification gave a standard that both rifle makers and ammunition makers must adhere to for a particular cartridge.

BUT!!. If you look at the SAAMI prints many of them show the max ammo size can be larger than the minimum chamber size. Hornady is producing ammo larger than the minimum chamber size. The go gauge passes but the ammo will not fit in the chamber.
SAAMI, if they ARE the standard setting organization should change their number to correct that. If I was making ammo I would make sure the ammo is .001 under the minimum chamber size or "go"gauge.

Look real close at the SAAMI grendel chamber print, there is no freebore. .2644 to .2617 in .153" directly in front of the case and cone.
The cone is .017" long, after that should be a freebore .264- .2645 in dia. In the Grendel where the normal freebore would start the throat starts, gets smaller from .2644 to .2617 in .153" then changes angle to 1.5 degree.
If any part of the bullet sticking out of the case is .264 dia it will hit the sides of the compound throat. ED-explained below
Edited for more accurate explanation. If we calc out the angle of the .2644-.2617 area the point where that area becomes smaller than the bullet dia is .0226 in front of the cone or 1.5498" from the base. If the dia of any bullet is .264" and in front of 1.5498 it will be jammed into the lands or in this case the compound throat.
After checking the Hornady 123 Amax ammo I'm betting the bullets will scuff in every Grendel chamber not just the Satern chambers. Color the bullets with a sharpie and chamber them.