stancrist said:

Plus, the shape of the GP bullet is clearly not the same as that of the machined copper solid

Perhaps its more inline with the shape of the Tungsten AP projectile? 

https://imlive.s3.amazonaws.com/Federal%20Government/ID5041233544762060809463086928281536454/NGSW_Industry_Questions_&_Comments_152_-_281_Part_2.pdf

Then theres this:

https://soldiersystems.net/2018/10/04/161298/

* General Purpose (GP) per Drawing titled “6.8MM GENERAL PURPOSE (GP)”. The GP cartridge provides all-purpose solutions for combat, limited training, and basic qualification.
* Surrogate per Drawing 13072652. The surrogate cartridge is designed to mimic the behavior of combat projectiles from a weapon design standpoint. Surrogate projectiles may not be completely representative of the final combat ammunition configuration which are expected to vary during development. Surrogates are intended to be a close replacement shape of the final combat rounds.

This seems to suggest Surrogate is different then the GP projectile in weight and or shape.

Which leaves open the chance that AP load is fairly different in weight and shape then the GP EPR, as if they were more or less the same there would be no need for both to be tested. 

gatnerd said:

* Surrogate per Drawing 13072652. ... Surrogates are intended to be a close replacement shape of the final combat rounds.

This seems to suggest Surrogate is different then the GP projectile in weight and or shape.

Concur re shape.  The "Industry Questions & Comments" explicitly states that the GP and SP projectiles are different in shape.

However, I do not see anything in the items you quoted which indicates that the GP and SP bullets necessarily differ in weight.

YouTube

SIG NGSW 6.8mm Expert Interview

Full interview with Jason St. John the SIG expert on the MG68 Next Generation Squad Weapon. This future machine gun with replace the US Army's 5.56mm M249 Sq...

Notes:

-SIG has a GPMG version with QCB and handle to allow barrel changes like 240 in the field

-8:50 shows good cutaway of how recoil buffer works

-Claim case can go 2x the pressure of brass, only ~10% of its extra capability used (ability to grow in power for future needs)

-Suggest weapon could also handle even higher pressures without needing big changes

-6.8lmg backwards compatible with 6.5C and 7.62x51 with just a barrel change

-Equal or less recoil to M249 SAW

....

The ability to grow in power within the current cartridge is very interesting, and does speak to the SIG cases advantage over the TV case. 

....

TV is 'moderate' (below 80k) pressure + longer barrel; sensible today but leaves limited room for growth in performance in coming decades. 

...

SIG 80kpsi + short barrel leaves option of say, defeating future Level V armor by going with a longer barrel to increase velocity, and 'Level VI' armor by going Long Barrel + Higher (100kpsi +) in the future. Of course thats predicated on a similarly linear development in armor tech. 

But so far this was my favorite video on SIG's NGSW because it got more into the smaller details that we'd be interested in. 

Am i right in thinking that polymer case requires thicker walls, and combining it with 80k PSI would need even thicker walls?

If one had a polymer 80k 5.7x28, would it be substantially larger than the original 5.7?

SIG composite case is steel reinforced only on the very bottom,they can apply the same reinforcement to the plastic case from TVC from what i understand about high pressure cases its about supporting the unsuported/unencased part between chamber end and boltface hence the steel case heads. Of course, then there is the question of how well the plastics can hold/ seal the steel case head.

Mr. T (MrT4) said:

SIG composite case is steel reinforced only on the very bottom,they can apply the same reinforcement to the plastic case from TVC from what i understand about high pressure cases its about supporting the unsuported/unencased part between chamber end and boltface hence the steel case heads. Of course, then there is the question of how well the plastics can hold/ seal the steel case head.

True Velocity Composite case proposed for the NGSW program was able to withstand bore obstruction overpressure without any damage.

I think sig is talking bigger than they can back up.

Especially about what TV can or cannot do.

And being incredibly blunt, sig NEEDS to do this because their ammo concept is by far the worst, most expensive, and will create the most logistical issues due to the loss of round count per standard pallet!

The best option right now is probably TV ammo in sig guns or Textron.

roguetechie said:

[SIG 6.8 ammo] will create the most logistical issues due to the loss of round count per standard pallet!

How so?

The increased base diameter, very simple answer.

???  SIG base diameter is the same as TV.  Both are smaller than Textron CT case diameter.