Its not a 556, its the 6.5 Grendel. A SAAMI "Callout" is the dimensions required to classify a chamber as a SAAMI.
There are 3 popular variations of the 6.5 Grendel: The Original Grendel with the .300" compound throat, the 264LBC with a straight throat and a .295" neck, and the Grendel II with the straight .300" throat.
The 264LBC is not considered a "SAAMI Grendel" because there is a change in the dimensions of the chamber. The throat however, has nothing to do with headspace and just dictates how far out a bullet may be loaded . There is a lot of heat and incorrect information being spread on the 65grendel.com forum bout how "the Grendel II isnt a SAAMI Grendel because it doesnt have the compound throat." This is wrong but i need an official email from a SAAMI board member to prove my point and have backing for it.
TGG