So further to your comments, I understand that the 6.5 mm Creedmoor with a 6-7 gram EPR bullet is fired at a higher chamber pressure (70,000 psi). This enables it to use standard brass (making a polymer case version potentially possible).. I further understand that the overall configuration delivers comparable penetration to 6.8x51 mm out of 14/16” barrels. (Anecdotally I am hearing Level IV defeat, but to 400 metres instead of 500 metres - SIG doesn’t quite do it at 600 metres, especially with shorter barrels). 
 

So with high pressure 6.5x48 mm Creedmoor SP you are delivering overmatch in a lighter package (18-19 grams versus 21-22 grams) with a cartridge that is also much easier and less expensive to produce. Because it runs less hot with lower pressures, barrel and parts wear should also be reduced. 
 

If it works as I think it does, it may be a much better overall solution than 6.8x51 mm. Indeed 6.5x48 Creedmoor SP could yet become the NGSW standard. 

6.5 Creedmoor is a 62kpsi cartridge, and 6.8 NGSW is much faster out of 14-16” barrels due to being a 75-80kpsi cartridge.

The 6.5 Creedmoor case has less taper and a sharper shoulder than the 7.62x51 and 260 Rem.  Have these presented any problems in weapons function?

gatnerd said:

6.5 Creedmoor is a 62kpsi cartridge, and 6.8 NGSW is much faster out of 14-16” barrels due to being a 75-80kpsi cartridge.

The commercially available 6.5x48 mm Creedmoor cartridge shoots at 62,000 psi. But insiders tell me that the military version developed by SOCOM and incorporating an EPR bullet, shoots at 65,000-70,000 psi. The 6.5 mm bullet mass is 105 grains (6-7 grams) versus 120 grains (8-9 grams) for 6.8x51 NGSM. So performance should be almost as good. 

I am going to suggest that 6.5x48 mm Creedmoor SP penetrates Level IV plate at 300 metres, while SIG's 6.8 mm cartridge does so at 500 metres. If the 6.5x48 mm Creedmoor can deliver this level of performance using standard brass in a cartridge with a lower weight, and less recoil, it will offer a better set of compromises that could weigh in its favour. At the very least, I expect the excellent work being done by SOCOM to influence the final NGSW standard. 

Also, SOCOM is also fielding a weapon in 6x39 mm ARC, also with an EPR bullet. If you extend the ARC case to 45 mm and put more powder behind it, it may be possible to deliver respectable Level IV penetration in a cartridge close in weight to 5.56 mm but that overmatches 7.62 mm beyond 300 metres.  

@nincomp

No issues with belted 6.5 mm CM.

Guardsman26 said:

I expect the excellent work being done by SOCOM to influence the final NGSW standard. 

In what way(s)?

Guardsman26 said:

Also, SOCOM is also fielding a weapon in 6x39 mm ARC, also with an EPR bullet. If you extend the ARC case to 45 mm and put more powder behind it, it may be possible to deliver respectable Level IV penetration in a cartridge close in weight to 5.56 mm...

A 6x45 ARC would weigh ~16 grams.  That is a little closer to 5.56x45 NATO than 6.5x48 CM at 18 grams, but not by very much.

And unless completely new NGSW-R and NGSW-AR of reduced size would be developed, I'm doubtful it would be worth doing.

I think there is a growing consensus among the US Army user community (especially SOCOM) that NGSW has “thrown the baby out with the bath water.” By this I mean that by setting such a high requirement for Level IV armour defeat at 600 metres, it has re-introduced all of the disadvantages that led to 7.62x51 mm NATO being replaced by 5.56x45 mm NATO. Chief among these is that soldiers will have less available rounds for a given weight of ammunition carried. The cost implications (four-piece (ammunition, increased barrel and parts wear, expensive exotic fire control unit etc.) ,   the logistical implications, the training burden, and most significant of all, the dismounted infantry soldier’s weight burden will increase. So, rightly, SOCOM, is trying to get RDECOM to refocus on priorities other than target defeat at 600 metres. Maybe 300 metres is enough, if it overcomes some of the other disadvantages. I believe SOCOM will bring a healthy dose of realism back into the program so that NGSW makes sense. 

Guardsman26 said:

I think there is a growing consensus among the US Army user community (especially SOCOM) that NGSW has “thrown the baby out with the bath water.” By this I mean that by setting such a high requirement for Level IV armour defeat at 600 metres, it has re-introduced all of the disadvantages that led to 7.62x51 mm NATO being replaced by 5.56x45 mm NATO. Chief among these is that soldiers will have less available rounds for a given weight of ammunition carried. The cost implications (four-piece (ammunition, increased barrel and parts wear, expensive exotic fire control unit etc.) ,   the logistical implications, the training burden, and most significant of all, the dismounted infantry soldier’s weight burden will increase. So, rightly, SOCOM, is trying to get RDECOM to refocus on priorities other than target defeat at 600 metres. Maybe 300 metres is enough, if it overcomes some of the other disadvantages. I believe SOCOM will bring a healthy dose of realism back into the program so that NGSW makes sense. 

one can hope. I also wonder what the lessons from UA will be. Back to maxims in 7.62x54R?

Guardsman26 said:

I believe SOCOM will bring a healthy dose of realism back into the program so that NGSW makes sense.

Sorry, but I don't see that happening.  SOCOM wants 6.5x48 CM, a cartridge which is as big -- and nearly as heavy -- as 6.8x51 SIG. 

And switching to 6.5x48 CM would give zero reduction in size or weight of the weapons, and zero increase in magazine capacity.

If 6.5x48 mm CM is fired at standard pressures, with a performance more or less equivalent to 6.8x51 mm, it would allow a polymer version to be created. This would offer a significant weight saving. But as you note, it would not increase total rounds carried relative to 6.8x51 mm. 

However, more fundamentally,, SOCOM is trying to drive a more realistic requirement. This is significant, but whether it leads to lead to an ammunition derived from 5.56 mm or 7.62 mm, hopefully it will not increase the weight burden.. 

Guardsman26 said:

If 6.5x48 mm CM is fired at standard pressures, with a performance more or less equivalent to 6.8x51 mm, it would allow a polymer version to be created. This would offer a significant weight saving.

I mean, they had a polymer cased cartridge that fired at standard(ish) pressure of 65kpsi - the 6.8TV. They went with the high pressure, heavier metallic case of the SIG. 

At 62kpsi (or even 65) the velocity is much lower - about 300fps lower - then the 6.8 SIG from a 16" barrel. 

6.5C is a fine cartridge, but I dont see it either providing comparable performance to 6.8 NGSW, or solving most of the issues (weight, weapon weight, limited mag capacity) with NGSW.