I do admit it's incredibly 'out there'. Back in the 70s Olin/Winchester had developed sabots capable of withstanding 70kPSI, and sending a tungsten projectile of similar diameter and weight at over 1200 m/s. Maybe one could engineer a sabot with 2020s technology, to withstand higher pressures if you do some concessions on how long it has to withstand them. And perhaps gain twist rifling could let you do some concessions on how much rotational straing it needs to handle.

A more conservative carbine approach is firing the earlier mentioned 6mm 81gr EPR through a 6mm bore at a similar 700-ish m/s, and accept that you won't have an easy subsonic option at a magazine swap.

gatnerd said:

But having a lighter caliber for the carbine still provides a number of advantages  -Reduced ammo weight -Greater magazine capacity -Reduced recoil / increased FA controllability  -Lighter rifle weight

That's right, but you also have to produce and to field 2 different ammo... 

I don't see the US Army replacing the 7.62 mm NATO with the .264 LICC, and the 5.56 mm with a high velocity 6 mm. 

Both cartridges will have similar volume and energy, the 6 mm version being slightly lighter (3 grams less?), with better long-range ballistics (which is not really what you want for you IW), but less barrel life and more difficulties to produce AP/I/T ammo. 

Another scenario would be that True Velocity win the 6.8 mm conversion program for the M240 and that the US Army adopt the 6.8 mm TVC cartridge for the M240, M250 and M7.

The M4 remains in service, but with a new 5.56 mm load.

mpopenker said:

EmericD said: If the US wasn't the US, they could ask SIG to make a +P version of the .17 Fireball in order to launch a 30 gr EPR bullet at 1000 m/s from a 13" carbine and a little less from the 8" derived PDW (or a .20 +P VarTag, with a 45-50 gr bullet at the same MV). I think firing such a round with some water in the bore (capillary effect is a bitch with smallish bores) could be a very interesting experience.

Water obstruction was one of the first argument thrown against the 5.56 mm during the '50s, and now most 5.56 mm rifles have "Over The Beach" firing capability.

But I agree that barrel obstruction with such diminutive cartridge, operating at 75,000 - 80,000 PSI, is probably something to consider (a single drop of oil inside the barrel, after cleaning the rifle, could probably be enough to make the gun unsafe), and barrel life will probably be miserable.

stancrist said:

#1 expresses the desirability for using minimum resources for rifle ammo, but it cites no evidence from the current HIC which substantiates there is need to do so.

So, what would be an "evidence"?

The worldwide difficulty to produce enough propellant and resulting ammo shortage, the 38% cost increase in cartridge components compared to 2019, or the structural debt of western budgets?

stancrist said:

#2 addresses the problem of detection of laser designators, but it does not in any way show that this has been an issue with the muzzle flash and sound of rifle fire.

Shooting a rifle is producing a large IR flash that is easier to detect than a laser designator using a specific IR band.

VPMudde said:

I do admit it's incredibly 'out there'. Back in the 70s Olin/Winchester had developed sabots capable of withstanding 70kPSI, and sending a tungsten projectile of similar diameter and weight at over 1200 m/s.

And sometimes the tungsten projectile decided to exit the barrel before reaching the gun muzzle, and the L/D of the XM948 projectile is "only" around 4 if I remember well...

VPMudde said:

Maybe one could engineer a sabot with 2020s technology, to withstand higher pressures if you do some concessions on how long it has to withstand them. And perhaps gain twist rifling could let you do some concessions on how much rotational strain it needs to handle.

You still have to be sure that the bullet won't slip into the sabot.

Remember that the US had to replace the M855A1 bismuth core with a copper core, because of low adherence of the bismuth core to the jacket at high temperature, and the L/D of the M855A1 is just 4.5.

VPMudde said:

A more conservative carbine approach is firing the earlier mentioned 6mm 81gr EPR through a 6mm bore at a similar 700-ish m/s, and accept that you won't have an easy subsonic option at a magazine swap.

That's a point I don't understand. 

The L/D of this bullet should already be around 5, requiring at least a 1-in-7" twist, so you could use a 1-in-6" or even a 1-in-5", and be OK for a subsonic load?

I already fired some .30" bullets at 780 m/s from a 1-in-7" twist, the Sg was above 3 but the bullet flew well up to more than 3500 m.

EmericD said:

but you also have to produce and to field 2 different ammo...

I'd call that a non-issue, seeing as how two different calibers have been produced and fielded by the US Army for the last six decades.

EmericD said:

I don't see the US Army replacing the 7.62 mm NATO with the .264 LICC, and the 5.56 mm with a high velocity 6 mm.

Nor do I.  But perhaps you do not understand that this thread is not about reality.  It is about a fictional scenario.

EmericD said:

Both cartridges will have similar volume and energy, the 6 mm version being slightly lighter (3 grams less?), with better long-range ballistics (which is not really what you want for you IW)...

That depends upon which "you" you're talking about.  Not everybody shares your opinion.  Some people do think that the IW really should have better long range ballistics.

EmericD said:

       stancrist said: #1 expresses the desirability for using minimum resources for rifle ammo, but it cites no evidence from the current HIC which substantiates there is need to do so.

So, what would be an "evidence"? The worldwide difficulty to produce enough propellant and resulting ammo shortage, the 38% cost increase in cartridge components compared to 2019, or the structural debt of western budgets?

Is there an ammo shortage in the current HIC?  What is the difference in cost of cartridge components between 5.56x45 and 7.62x51?  Is the cost difference even discernible in western budgets?

Considering the published price of 6.8x51 NGSW ammo, it seems clear that the US Army does not share your concern about the cost of rifle cartridges.

EmericD said:

       stancrist said: #2 addresses the problem of detection of laser designators, but it does not in any way show that this has been an issue with the muzzle flash and sound of rifle fire.

Shooting a rifle is producing a large IR flash that is easier to detect than a laser designator using a specific IR band.

You are still not providing any evidence that the current HIC shows the flash and sound of rifle fire to be a problem.  

stancrist said:

I'd call that a non-issue, seeing as how two different calibers have been produced and fielded by the US Army for the last six decades.

Well, in France we even managed to field 5.56 mm M193 family ammo (for the FAMAS F1) along 5.56 mm SS-109 family ammo (for the FAMAS Surbaissé / FELIN), but I won't call that a "non-issue".

Yes, it was done, it could be done, but it's far from ideal.

stancrist said:

That depends upon which "you" you're talking about. Not everybody shares your opinion. Some people do think that the IW really should have better long range ballistics.

I agree that some people think that the IW should have better long range ballistics than the current 5.56 mm, but I seriously doubt that some people think that the IW should have better long range ballistics than the LMG.

stancrist said:

Is there an ammo shortage in the current HIC?

Yes.

stancrist said:

What is the difference in cost of cartridge components between 5.56x45 and 7.62x51?

The 7.62 mm NATO is twice as expensive as the 5.56 mm.

stancrist said:

Considering the published price of 6.8x51 NGSW ammo, it seems clear that the US Army does not share your concern about the cost of rifle cartridges.

Great for the US Army and for the US budget, let's see what's going to happened in 2025.

In terms of .264 LICC and the current state of Shell Shock cases / thin wall stainless steel, do you know how much case weight savings they offer?

Previously SS's 9mm cases with the aluminum base / steel body had a 50% case weight reduction vs brass. However with their new scalloped steel base + thin wall steel body, I imagine the savings is no longer 50%?

Trying to get a decent estimate for the weights for my 2 caliber configuration.