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)

I used DSEI as an opportunity to find out what is going on with NGSW. 

A number of engineering change proposals for the XM7 have been submitted and are being implemented in the latest LRIP batch. Surprisingly, one of these is the elimination of the forward assist. Eugene Stoner is on film somewhere saying that this is the most unnecessary modification ever made to the M-16  / M-4, and I agree wholeheartedly. See image. 

More importantly, previously reported issues about ammunition accuracy and reliability were a function of the EPR-style bullet furnished by the US Government. This has been re-designed. In addition to a revised 6.8 mm projectile, new 6.5 mm and 6.35 mm bullets also seem likely to be evaluated. So the final calibre could actually change. I am sure that ballistic calculator aficionados among members of this forum will be able to work out the increased penetration offered by a slightly smaller bullet fired at a slightly higher velocity. It's not clear as to what range Level IV penetration will be be achieved. I expect it to be 300 metres, but that is purely a guess on my behalf. 

The most important thing about Phase 2 testing is that it has validated SIG's hybrid cartridge concept. The weapon is more than able to cope with higher chamber pressures. Standard loads will be fired at 90,000 psi. Proof loads will be fired at 120,000 psi, according to a source I spoke to. I was told that a significant  increase in chamber pressure has been common to all small arms development milestones for 200 years. 

The US Army has made it clear that it deliberately set the bar very high with NGSW. It wanted to see the art of the possible, which SIG has amply demonstrated. Now that there is certainty around the ammunition technology and performance envelope, production guns and ammo will be fine tuned to maximise range, terminal effectiveness, reliability and longevity. 

SIG's approach has also been validated by SOCOM's 6.5  mm LICC cartridge developed by FN America. This is also fired at a much higher pressure than legacy ammunition, believed to be 70,000 psi. They are also achieving good results at longer ranges. LICC may ultimately be a science project that goes nowhere, but it seems likely to influence the final NGSW standard.

As reported elsewhere, BAE Systems at Radway Green has its one-piece steel cartridge technology. This can also sustain much higher chamber pressures. It is supposed to have a unique means of lubrication to prevent the case sticking in the chamber after being fired.

Despite my earlier misgivings, I am quietly optimistic about NGSW. If it can deliver the desired step-change in performance while reducing overall weight relative to 7.62 mm, then it will be worth the effort of switching.  

Guardsman26 said:

I am quietly optimistic about NGSW. If it can deliver the desired step-change in performance while reducing overall weight relative to 7.62 mm, then it will be worth the effort of switching.

The trouble is, NGSW is meant to switch from 5.56mm, not 7.62mm.

Switching to 6.8mm will increase overall weight relative to 5.56mm.*

*Worse yet, the switch from 30-rd mags to 20-rd mags will greatly reduce combat endurance.

Saint Kyle of Kenosha demonstrated that Saint Eugene was wrong about the forward assist.

Also wondering if we're going to see some experiments with high-pressure 5.56x45. Just drop in a new bolt, and possibly buffer and barrel, would be very appealing. 70+ grain EPR at 80,000 psi could be interesting.

Apologies if this has been discussed before previously, has consideration been given to the possibility of mixing up 6.8 mm ammunition with 7.62 NATO when it gets into regular use?

From inspection of the cartridge comparison picture, it looks like the action probably wouldn't close if 6.8 was loaded in a 7.62 NATO weapon due to the shoulder position (relying on out of battery safety as well), however would 7.62 NATO chamber and fire in a 6.8 (possibly forced closed with the forward assist) with potentially catastrophic results?

I do not recall seeing the subject ever having been discussed in this forum, nor do I know if 7.62 NATO can chamber and fire in a 6.8 NGSW weapon.  Perhaps someone else knows.

PRM2 said:

From inspection of the cartridge comparison picture, it looks like the action probably wouldn't close if 6.8 was loaded in a 7.62 NATO weapon due to the shoulder position (relying on out of battery safety as well), however would 7.62 NATO chamber and fire in a 6.8 (possibly forced closed with the forward assist) with potentially catastrophic results?

Chambering a 7.62 mm NATO cartridge into a 6.8x51 mm weapon seems as easy as chambering a 7.62 mm NATO into a 7-08, a 260 Remington or a 243 Winchester weapon.

Indeed NGSW 6.8mm for full sized battle rifle like Ar10, FN FAL , SCAR H  kinda nonsense it they really meant to replace any 5.56 arms

The probable mitigation with that list of cartridges, plus 6.5 Creedmoor, is that they are likely to be used by specialists, who would be more likely to catch the potential 'Murphy'. The likelihood of potential miss-chambering would massively increase if 6.8x51 weapons were mixed with 7.62 NATO weapons in more general usage, unless as many people suspect, 6.8x51 ends up being used primarily in the DMR only as a specialised anti-armour round. 

It would be an interesting test to do with a 7.62 NATO drill round, or in the unlikely event of being allowed to potentially write off an expensive new rifle, do a remote test with a live round!