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. 

gatnerd said:

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.

You are absolutely right.  Because of its size, 6.5 CM simply cannot solve any of those NGSW issues. 

And even with polymer cases, 6.5 CM ammo will still be heavier than 5.56 NATO brass-cased ammo.

Let's be clear here, in order to deliver Level IV body armour defeat at 600 metres, NGSW presents several critical disadvantages: 

• The specification is identical to .300 Winchester magnum which is a sniping cartridge
• In order to avoid using a larger cartridge that would increase ammunition weight, the performance is packaged in 7.62x51 mm size case
•  This in turn results in chamber pressures of 80,000 psi
• As a result of this, heat builds-up in the weapon more quickly, reducing barrel and parts life, and battlefield reliability.
• It also introduces significant recoil, making automatic fire more difficult to control.
• Developing a weapon able to deliver this level of performance requires 4-piece ammunition. This is more complex and costly to produce.
• 6.8x51 mm reduces the amount of ammunition that can be carried for a given weight - this reduces hit probability
• Overall system cost is likely to be significantly greater even before you add the XM157 FCU optic and integral suppressor
• Overall system increases the logistics burden
• Overall system increases the training burden 
• Overall system increases the dismounted soldier's weight burden
• All of this extra long-range performance comes at the expense of 0-300 metres performance - and still this remains the critical range envelope

The question raised by 6.8x51 mm NGSW is how important is the need to engage enemy targets at 600 metres? And do the benefits of being able to do so outweigh the clear disadvantages. How often will we need to engage enemies at this range, and how convinced are we that 6.8x51 mm NGSW will be effective versus other weapons we might employ, such as 60 mm, 81 mm and, 120 mm mortars? 

As someone who has worked in military equipment acquisition for 20 years as well as being a former infantry officer, to me NGSW is redolent of very bad requirements writing. I was brought-up on 7.62 mm. But I also have operational experience of 5.56 mm. An infantry section / squad shooting 5.56 mm is consistently better able to to achieve target effect than the same group using 7.62 mm. NGSW isn't just a reintroduction of 7.62 mm NATO, it is a much more powerful round, so I don't expect shooting results to be significantly better even with an optic. 

For these reasons, I believe we need NGSW to have a lesser requirement - arguably to defeat body armour to 300 metres. This will result in an ammo that is more capable than 7.62 mm, but lighter, and without the cost and logistical issues of 6.8 mm NGSW ammunition. 

It appears that SOCOM shares this view. And we are NOT talking about standard 6.5 mm Creedmoor - we are talking about a new specification based on that cartridge fired at a higher chamber pressure so it delivers increased penetration out of shorter barrels. (Gatnerd, your 6.5 mm chart is meaningless in this context).

Let's call SOCOM's 6.5 mm round 6.5 mm Creedmoor SP.  Is it optimised to blanche the conflicting requirements for NGSW? No. Because it still reduces the number of rounds carried for a given weight. Would I prefer a 6 mm round fired at the highest possible velocity in the smallest possible case? Yes. But, if the requirement demands Level IV defeat, and this mandates 6.5 or 6.8 mm, we may have to accept certain disadvantages.  What we can't do is destroy logistical efficiency, increase the training burden, increase the weight burden by adopting a more expensive weapon with more expensive ammo and that has a shorter life. 

Finally, for the sake of clarity, TV polymer ammo is not the only game in town. MAC's technology, which was bought by Nammo, has a working polymer 6.5 mm case that supports 60,000-70,000 psi. 

Guardsman26 said:

It appears that SOCOM shares this view. And we are NOT talking about standard 6.5 mm Creedmoor - we are talking about a new specification based on that cartridge fired at a higher chamber pressure so it delivers increased penetration out of shorter barrels.

I mean, if its a meaningfully higher pressure 6.5C (65-70kpsi) then its realllly getting to a near pointless distinction vs 6.8 NGSW in terms of benefits.

-Comparable/identical weapon weight

-Identical ammo loadout / magazine capacity / belt capacity 

-Identical cartridge volume 

-Still High recoil 

-Higher heat and wear then 7.62 due to increased pressure and narrower swept bore volume 

....

MAC LLC's polymer ammo has a brass base (the heaviest part of the cartridge), making weight savings pretty marginal. 

If SIG's 6.8 is 22gr, the polymer 6.5 would need to be 18.75g in order to shave off 1lb from a 140rd (7x20rd mag) loadout vs 6.8. Itself a rather meh improvement. 

I don't really see 6.5C/6.5SP as much if any of an improvement. If anything it just seems like having all the drawbacks of NGSW with less performance. 

Guardsman26 said:

NGSW presents several critical disadvantages: 

* 7.62x51 mm size case  

* It also introduces significant recoil, making automatic fire more difficult to control.

* 6.8x51 mm reduces the amount of ammunition that can be carried for a given weight - this reduces hit probability

* Overall system cost is likely to be significantly greater even before you add the XM157 FCU optic and integral suppressor

* Overall system increases the logistics burden

* Overall system increases the training burden 

* Overall system increases the dismounted soldier's weight burden

* All of this extra long-range performance comes at the expense of 0-300 metres performance - and still this remains the critical range envelope

The trouble is, all of the above disadvantages are also true of 6.5 Creedmoor guns and ammo.

Guardsman26 said:

The question raised by 6.8x51 mm NGSW is how important is the need to engage enemy targets at 600 metres? And do the benefits of being able to do so outweigh the clear disadvantages? How often will we need to engage enemies at this range?

As I recall, not too long ago GPC proponents in this forum were arguing that it was very important to be able to engage the enemy at 600-plus meters, and they insisted that the benefits very much outweighed the disadvantages.

Guardsman26 said:

For these reasons, I believe we need NGSW to have a lesser requirement - arguably to defeat body armour to 300 metres. This will result in an ammo that is more capable than 7.62 mm, but lighter, and without the cost and logistical issues of 6.8 mm NGSW ammunition. 

I agree.  Actually, I'm inclined to think that armor defeat range could be reduced to circa 100 meters.

Guardsman26 said:

Let's call SOCOM's 6.5 mm round 6.5 mm Creedmoor SP.  Is it optimised to blanche the conflicting requirements for NGSW? No. Because it still reduces the number of rounds carried for a given weight. Would I prefer a 6 mm round fired at the highest possible velocity in the smallest possible case? Yes. But, if the requirement demands Level IV defeat, and this mandates 6.5 or 6.8 mm, we may have to accept certain disadvantages.  What we can't do is destroy logistical efficiency, increase the training burden, increase the weight burden by adopting a more expensive weapon with more expensive ammo and that has a shorter life.

Except for the expensive ammo, it seems to me you'll also have pretty much all of those issues with 6.5 CM SP. 

Since the 6.8x51 weapons have already been developed, I am not sure that switching to 6.5x48 now is worth it.

There is utility in the GPC idea. But not in the form of making a smaller battle rifle cartridge. The Russians were onto the right idea back in the 80s/90s.  

6mm diameter, 77gr projectile at 3700FPS muzzle velocity.

The Firearm Blog

Modern Intermediate Full Power Calibers 019: The Russian 6x49mm Unified -

What happens when you take the two concepts of a traditional, full-power rifle and machine gun round, and a small-caliber, high-velocity round, and smash them together? You get one of the most extreme military small arms calibers ever developed, and one of the last small arms projects from the twilight of the Soviet Union. ... Read More ...

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That to me does a lot better at stepping up the power and range of the 5.56NATO, without getting to the point of excessive recoil. A lighter bullet going faster is easier on the shooter than a heavier bullet going slower. And modern metallurgy should keep us out of the undesirable barrel wear zone for longer.

A 100yrd penetration requirement is reasonable, I'd agree there.

If I recall correctly, GPC never had the goal of defeating body armor at enormous distances, most people here said that GPC was to be accurate and effective enough to pin down the enemy so that HE could do the job.

Emeric uses the 'suppression index' (or something like that) for the 'effectiveness' part. Maybe I'm wrong but body armor or not, if you are being (almost) hit by pills that can do more than a paper cut you're pinned down, and troops carrying HE can finish the job.

If you can pin down the enemy (relatively) far away then you have the additional option of calling the big guns.

renatohm said:

If I recall correctly, GPC never had the goal of defeating body armor at enormous distances...

That's true, but his questions are:

"How important is the need to engage enemy targets at 600 metres?"

and

"How often will we need to engage enemies at this range?"

Apsyda said:

The Russians were onto the right idea back in the 80s/90s.  

6mm diameter, 77gr projectile at 3700FPS muzzle velocity.