Is it a crack running around the "6.8x51" mark, and an ejector mark between the "BB" and the kangaroo?

The 95 SMK is 1.13 and that was what I used to design the wildcat. The CE 85 is 1.136" long so I'm sure i would work.

I knew the bolt strength was a problem for Grendel/ARC sized cartridges in an AR15 as soon as I saw one in late 2005. I didn't start machining bolts until 2009. The one on the far right was the first (800 series) to allow me to shoot a 6mmBRX in an AR15, 2nd from the right was the 750XD, it was also strong enough to use .473" cartridges in an AR15. The middle bolt is the Titan, I designed it specifically for the Grendel sized cartridges. 2nd from left is the 6.8 Superbolt made to work with standard AR barrel extensions but apx 13% stronger than a broach cut mil spec design.

If you look at the new Sig Spear LT the bolts look similar to the Titan but I'm not sure about lug length or exact OD of the head. There are several ways to increase the strength of a Grendel/ARC bolt  but it surprises me that a big company hasn't in 15 years. If Geissele continues working on the Grendel they may do it, they have already acknowledged the typical Grendel mag has issues and have manufactured their own.

Is "750XD" meaning "0.750 inch external diameter"?

Yes, same dia as a mil spec but the shape of the lugs increase the strength.

gatnerd said:

Seems Australia is developing its own 6.8x51 that does not use a 2 part case like SIG:

I wonder if they used the 277 Fury case shape?  Compared to a simple 277-308 wildcat, the Fury has less body taper, sharper shoulder and shorter neck.  This gives a slightly larger propellant capacity for a normal brass case.   Things could change if the case walls get thicker, of course.  It would be interesting to see how high the pressure could be pushed with an optimized brass or steel case.

nincomp said:

I wonder if they used the 277 Fury case shape

I'd be astounded if they weren't copying the same case dimensions given their goal to have some commonality with the US 6.8. 

The SIG Spear seems to have the ability to fire both standard ~60kpsi brass ammo as well as ~80kpsi hybrid ammo, at least from the few youtube clips we've seen.

I imagine the Australians are interested in a similar goal? This domestic case that they can make royalty free, that offers 'general purpose' performance, but retain the ability to fire high pressure US AP when the AUKUS balloon goes up?

All speculation on my part here, I just have that little insta post to go on. 

gatnerd said:

       nincomp said: I wonder if they used the 277 Fury case shape

I'd be astounded if they weren't copying the same case dimensions...

Ditto.

gatnerd said:

This domestic case that they can make royalty free, that offers 'general purpose' performance, but retain the ability to fire high pressure US AP when the AUKUS balloon goes up

Quite logical.

EmericD said:

Is it a crack running around the "6.8x51" mark, and an ejector mark between the "BB" and the kangaroo?

Concur on the mark between the "BB" and the kangaroo being an indentation from the ejector.

I think it is not a crack at the "6.8x51" mark, but perhaps an impression of the extractor groove on the bolt face.

stancrist said:

I think it is not a crack at the "6.8x51" mark, but perhaps an impression of the extractor groove on the bolt face.

Could be. My experience is that the extractor mark is generally found inside the groove, where the extractor try to shear the rim.

Looks like that this case is operating just below the threshold of catastrophic failure.

EmericD said:

My experience is that the extractor mark is generally found inside the groove, where the extractor try to shear the rim.

Not the groove in front of the cartridge case rim.  The groove / channel machined into the bolt, into which the extractor is fitted.

Although to make that shape of impression on the case head, the extractor channel bottom would have to be rounded instead of flat.