gatnerd said:

BAE seems to also have developed a specialized steel case design, possibly capable of higher (ngswish) pressures, and likely offering weight savings as well.

There reportedly is a weight savings, but the 5.56 / 7.62 ammo is not loaded to higher pressures.

BAE Systems providing lighter small arms ammunition to lower soldiers’ burden - EDR Magazine

But BAE may not be still pursuing this.  There is nothing on their site, and the article is 4 years old.

Looks like BAE may have changed to a one-piece steel case.  The article below is from 2 years ago.

The New Lightweight Bullet That Saves Fatigue ... And Fuel (forces.net)

did the 556 CT rounds offer any reduction in volume

what was the barrel length on the textron rifle. it looks like 16, and yet the round apparently didnt hit 80k PSI

with wider cartridges like the 6ARC, could you some how angle the magazine at 45 degrees to improve prone shooting?

and then somehow use a special feed ramp to get the bullets in barrel

smg762 said:

what was the barrel length on the textron rifle.

IDK.

smg762 said:

did the 556 CT rounds offer any reduction in volume

Here is a 2014 presentation of the 5.56 LSAT.  Main selling point was ammo weight reduction, largely for linked.  37% weight reduction, 12% for volume.

https://ndiastorage.blob.core.usgovcloudapi.net/ndia/2014/armaments/Wed16533_Shipley.pdf

Here's an unusual development:

https://soldiersystems.net/2023/09/18/army-marksmanship-unit-seeks-match-grade-6-8-ammo/

1a. BULLET: .277 dia. 135gr Sierra Match King OTM item or .277 dia 140gr Classic Hybrid Hunter Berger Bullet. End user will accept projectiles not listed if precision dispersion is better than specific projectiles/bullets listed. If an optional projectile is proposed the optional projectile cannot weigh more than 150gr.

....

1f. MUZZLE VELOCITY: ammunition must produce a velocity sufficient to provide Sub MOA accuracy from 16″ Bolt Action barrels & 1.15 MOA or less from semi auto barrels and maintain low SD of MV. Standard deviation of MV should be 13 fps +or- 1 fps (or less) for avg of 30 rds tested at 70F. Actual muzzle velocity shall be a mild velocity ammunition not to exceed 64 KSI from PSI test barrels at Hot temp ranges. 70F degree test barrel & ammunition should produce an avg of 58-59 KSI. End user not specifying specific muzzle velocity.

--> The use of lead projectiles is not entirely surprising for a match grade round. However the insistence that the round be of conventional pressure, rather then the 80kpsi we've seen in most talk about the NGSW, is quite interesting.

Whether this is just match ammo for competition / training, or an indication in a broader interest in a lower pressure loading for 6.8, remains to be seen.

gatnerd said:

The use of lead projectiles is not entirely surprising for a match grade round. However the insistence that the round be of conventional pressure, rather then the 80kpsi we've seen in most talk about the NGSW, is quite interesting. Whether this is just match ammo for competition / training, or an indication in a broader interest in a lower pressure loading for 6.8, remains to be seen.

Since it is for the AMU, that would seem to indicate it is for competition use.  There is no need for an 80kpsi loading just to punch holes in paper targets.

And if it would also be used by snipers for combat, precision shooting would be more important than the marginal increase in terminal effects of 80kpsi.

stancrist said:

And if it would also be used by snipers for combat, precision shooting would be more important than the marginal increase in terminal effects of 80kpsi.

The added velocity is also increasing the hit probability against moving targets, or targets located at unknown distance, not only terminal effects.

Maybe the base of a lead core OTM bullet is not strong enough to sustain the 80 kpsi of the full pressure hybrid ammo? The EPR bullet  is using a copper core inside the tombac jacket, which is less susceptible to plastically deform under high pressure load, and of course the all-copper "surrogate" bullet used also during the development phase was lead-free.

EmericD said:

Maybe the base of a lead core OTM bullet is not strong enough to sustain the 80 kpsi of the full pressure hybrid ammo?

You would know more about that possibility than I.  But whatever the cause, it appears that the 80 kpsi ammo does not meet AMU accuracy requirements.

As I recall, the 135gr Sierra MatchKing was used as a surrogate projectile during development of full-power ammo.  I imagine the AMU tested some of that.

BREAKING: SIG NGSW Prototype Unveiled To The Public (The Firearm Blog)