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6.5 Grendel bolts and extractors come from the .50 Beowulf, which came from the Colt 7.62x39 development.
If you track the history of the Colt 7.62x39 and how after-market companies basically ignored the R&D that went into it, thinking they could duplicate it with a simple enlarging of the bolt face, you see where the big boys get separated from the imitators really quick. Another major oversight that should have never happened was that a lot of the early and even later 7.62x39 after-market barrel makers used .308" bores instead of .310", which spikes the pressure during swaging of the bullet into the rifling.
Every after-market 7.62x39 manufacturer experienced unusually-high extractor and bolt lug failures as the norm for many years as a lot of them were using 8620 run through amateurish processes in the metallurgy.
The strongest AR15-sized bolt for 7.62x39 is made by LMT using an exotic aerospace AerMet alloy that costs twice as much per bolt as 9310 bolts cost. Even those break extractors because they use a .125" bolt face depth.
Since the 7.62x39 and 6.5 Grendel have larger diameter case rims and heads, that are thicker than 5.56 brass dimensions in the longitudinal axis (front-back), so the extractor geometry needs to be different to be able to correctly snap over during chambering, and not grind against the Inner Diameter of the barrel extension teeth.
With a .125" bolt face depth, there isn't enough back-to-front meat on the extractor lip.
The reason the AR10 can use a .125" face depth is that the extractors are much wider than AR15 extractors, so there is far more root strength for the extractor lip as part of the extractor body.
If you place an AR10 extractor next to an AR15 extractor, you'll immediately see what I'm talking about.
You would need to use an exotic alloy for the .125" bolt face depth extractors in order to gain the lip strength you need in the AR15, and tightly control the QC on them dimensionally with an unacceptably-high reject rate that would drive the costs even higher.
As to SAAMI, if you look at bolt guns, Contenders/Encores, and AR15s, the chamber depth is going to be based on the bolt face or breech design, balanced against case support at the web. The bolt gun extractors can have really thin lips because they don't have to extract during residual case obturation. It isn't arbitrary, but is based on several engineering considerations for the type of action and constraints of that particular action design.
.125" bolt face depth in the 6.5 Grendel and 6mm Grendel variants is just asking for broken extractors in the AR15. I've had a representative/dealer of BHW claim that they never had problems with .125" bolts breaking extractors, immediately followed by customers mentioning they experienced that exact problem with the .125" bolts in a ".264 LBC" chambered BHW barrel/bolt combo. You see it all the time with high volume 7.62x39 AR15s, where many shooters just accept that they need to carry spare bolts/extractors with them to the range.
The .125" bolt face depth is just a non-starter from a production standpoint for this reason.
If you track the history of the Colt 7.62x39 and how after-market companies basically ignored the R&D that went into it, thinking they could duplicate it with a simple enlarging of the bolt face, you see where the big boys get separated from the imitators really quick. Another major oversight that should have never happened was that a lot of the early and even later 7.62x39 after-market barrel makers used .308" bores instead of .310", which spikes the pressure during swaging of the bullet into the rifling.
Every after-market 7.62x39 manufacturer experienced unusually-high extractor and bolt lug failures as the norm for many years as a lot of them were using 8620 run through amateurish processes in the metallurgy.
The strongest AR15-sized bolt for 7.62x39 is made by LMT using an exotic aerospace AerMet alloy that costs twice as much per bolt as 9310 bolts cost. Even those break extractors because they use a .125" bolt face depth.
Since the 7.62x39 and 6.5 Grendel have larger diameter case rims and heads, that are thicker than 5.56 brass dimensions in the longitudinal axis (front-back), so the extractor geometry needs to be different to be able to correctly snap over during chambering, and not grind against the Inner Diameter of the barrel extension teeth.
With a .125" bolt face depth, there isn't enough back-to-front meat on the extractor lip.
The reason the AR10 can use a .125" face depth is that the extractors are much wider than AR15 extractors, so there is far more root strength for the extractor lip as part of the extractor body.
If you place an AR10 extractor next to an AR15 extractor, you'll immediately see what I'm talking about.
You would need to use an exotic alloy for the .125" bolt face depth extractors in order to gain the lip strength you need in the AR15, and tightly control the QC on them dimensionally with an unacceptably-high reject rate that would drive the costs even higher.
As to SAAMI, if you look at bolt guns, Contenders/Encores, and AR15s, the chamber depth is going to be based on the bolt face or breech design, balanced against case support at the web. The bolt gun extractors can have really thin lips because they don't have to extract during residual case obturation. It isn't arbitrary, but is based on several engineering considerations for the type of action and constraints of that particular action design.
.125" bolt face depth in the 6.5 Grendel and 6mm Grendel variants is just asking for broken extractors in the AR15. I've had a representative/dealer of BHW claim that they never had problems with .125" bolts breaking extractors, immediately followed by customers mentioning they experienced that exact problem with the .125" bolts in a ".264 LBC" chambered BHW barrel/bolt combo. You see it all the time with high volume 7.62x39 AR15s, where many shooters just accept that they need to carry spare bolts/extractors with them to the range.
The .125" bolt face depth is just a non-starter from a production standpoint for this reason.
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