Apsyda said:

The NGSW-AR...had to add a ridiculous bridging top rail because obviously there was little intercompany and intratrial communication as to how large the NGSW-FC optic would be.

The problem was not caused by a communication failure.  The bridging rail was needed because of a design flaw.

SIG had designed the LMG receiver so that only optics of certain dimensions and/or configurations could be used.

They knew a common FC optic would be used for both NGSW-R and -AR, and yet the SIG LMG was always seen fitted with a compact RDS, while the MCX Spear rifle usually had a much bigger LPVO.  Had the LMG been designed from the start to use the LPVO, the bridging rail likely would not have been needed.

YouTube

Overview of the Sig Sauer MCX Spear and XM250

Guns.com recently got to hang out with the folks from Sig Sauer and learned about the company's successful submission to the Army's Next Generation Squad Wea...

The folks at Battle Order have joined in.  

Nice, I love their posters. 

Watching this interview on SIG's 6.8 ammo:

https://youtu.be/yUVX_Bup0Ig?t=829

-Cases currently running "mid 70's" pressure aka around 75kpsi

-Claim the cases can be run to 120kpsi should the future require it

Man, thats worse than what I said.

Gotta love SIG.

I agree with Emeric. A standard pressure 6.8x51 mm NGSW must be VERY close in performance to 6.5 x 49 mm Creedmoor. I believe SOCOM has its own 6.5 mm EPR-style projectile that is identical to the 6.8 mm NGSW projectile in terms of profile, but weighs less - 6-7 grams. A 6.5x49 mm CM fired at standard pressure (55,000-60,000 psi) has the advantage of weighing 18-19 grams versus SIG's 6.8x51 mm cartridge at 21-22 grams. You can also use the 6.5 CM lower chamber pressure to engineer a polymer case to deliver a lower weight cartridge, close to 16 grams. 

Emeric, I am not a ballistician, but what do you estimate would be the chamber pressure of a 6-7 gram 6.5 mm bullet fired from a 6.5x49 mm Creedmoor cartridge at 3,000 fps / 914 mps? 

Second pint, popped primers with FN SCAR are due to some manufacturers using soft metal primers, not military spec ones. SCAR is designed to meet very demanding military standards. It is designed to achieve 1 mm of penetration for military spec 7.62 mm and 5.56 mm cartridge primers. With soft primers it is achieving 1.5 mm or 2 mm. The latter is obviously too much. I know this, because I was on the range with FN today, and asked their technical lead. (I was firing Evolys in5.56 mm and 7.62 mm ). 

Guardsman26 said:

I agree with Emeric. A standard pressure 6.8x51 mm NGSW must be VERY close in performance to 6.5 x 49 mm Creedmoor. I believe SOCOM has its own 6.5 mm EPR-style projectile...

Upon what do you base that belief?  Emeric said that SOCOM does not use EPR projectiles.

I believe SOCOM is experimenting with EPR projectiles.

That does not answer my question, but I would be extremely surprised if SOCOM is not developing both EPR and Tracer rounds, as they will be needed for the 6.5 CM LMG.

I wonder if there will be issues with developing a tracer that will match the trajectory of the high-BC bullets normally used in the 6.5 Creedmoor (and 6mm ARC, for that matter).   It may be easier to match the trajectory of less-dense EPR bullets than to match lead-cored ones.

 It has occurred to me that the mediocre BC of bullets used in cartridges like the 280 British and 6mm SAW might have had more to do with limitations of tracer rounds than ball ammo.