Harrison Beene (harrisonbeen) said:

I still have 2 of my own 6mm cake(TAC6) rifles based on a 6.8 case and built on the LWRC SIX8 platform.  18" barrels, 90 ScenarL-95SMK bullets at 2800fps.

We recently tried a LMT rifle chambered for the 6 mm ARC, and measured only ~780 m/s (2560 fps) with the 105 gr BTHP from a 18" barrel (2070 J at the muzzle vs. 2120 for your 6 mm SPC).

It seems that the AR-15 bolt diameter is a real limiting factor, and increasing the case diameter should be followed by reducing the cartridge MAP, so no real benefits at the end of the day.

Here, we are still testing the technical "bricks" to make a neckless 5.56 mm NATO.

We tested a 64 gr solid bullet with a C7 BC of 0.208, but the accuracy wasn't really good due to the large initial yaw of the design. We changed the boat-tail shape, the C7 BC dropped to 0.201 but the accuracy issues were solved. We were able to launch this bullet at 945 m/s from a 18" Mk12 rifle. The case was a regular 5.56x45 mm, but trimmed to 40.5 - 40.7 mm.

This bullet was lathe-turned but we started to study cold-forming, when the need for "increased penetration against ceramic armor" emerged, and derailed this effort.

So, now we are going to scale-up this bullet to .264", with a long steel core, and load this bullet in a high capacity .260 Remington steel case. The case capacity is ~20% higher than brass case (3.6 cm^3 excluding the neck and half the shoulder), weight is only 7.1 g, and we should be able to reach >970 m/s from a 18" barrel and with a bullet in the 95 - 100 gr class (for a cartridge mass around 17.0 - 17.4 g).

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

https://www.instagram.com/p/Cn26BxHvYDH/?igshid=YmMyMTA2M2Y%3D

Harrison Beene (harrisonbeen) said:

I still have 2 of my own 6mm cake(TAC6) rifles based on a 6.8 case and built on the LWRC SIX8 platform.  18" barrels, 90 ScenarL-95SMK bullets at 2800fps. For all practical purposes it is a 30 Herrett necked down to 6mm using 6.8 cases trimmed back to 1.59" but loaded to 2.3". Mag cap is 30 rnds

Super cool. Thats very close to the dream 6mm CAKE AR12 right there.

Does the round have enough space to fire the 88gr Cutting Edge? 

https://cuttingedgebullets.com/243-6mm-88gr-mth-match-tactical-hunting

Thats the projectile I've been thinking would be a good off the shelf surrogate for a future EPR style lead free VLD.

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.