stancrist said:

Basis for that conclusion?

1- Small arms ammo are secondary factors on the HIC battlefield, you should dedicate minimal amount of resources to produce them because you are going to fire billions of them. Any resource not used by small-arms ammo (like money) will be used more efficiently on bigger systems.

2- Avoiding detection is the best way to avoid being killed, so if the enemy can't easily detect you with IR or acoustic devices when you are returning fire, you could expect to live longer. The number of ground or aerial sensors deployed in a HIC battlefield is outstanding, I've been told that people deploying laser designators using an uncommon bandwidth for "stealthy operation" were "instantly" detected and engaged with 120 mm mortars in less than 3 minutes.

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.

I agree with both of those points, but neither one answers my question: 

How is the current HIC showing that rifle ammo needs to use the smallest amount of resources, and produce a muzzle report (flash & sound) as low as possible?

#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.

#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.

EmericD said:

Problem is, if you're the US, you're not going to develop a "2 caliber system" unless you screw your "one caliber system".

Quite true.  The premise of the OP -- adopting a two-caliber system at the squad level -- is contrary to US Army history.

But it may not be an impossibility, use of two calibers at squad level having been done on a limited basis in times past.

For the high end I squeezed down a .243 winchester: 6.8 SPC shell shock case necked down to 6mm, loaded with an 81gr ~0.25 G7 EPR to an overal length of 64mm. At a tad over 80kPSI and a 420mm barrel this gives you a 1000m/s MV, 600m fragmentation range, 1200m supersonic range. 

For the low end I was inspired by the AK47 being considered a submachine gun: 5.56mm 74gr ~0.25 G7 EPR sat in a 7.62mm sabot, launched out of a .30 carbine shell shock case loaded to >80kPSI through a 250mm barrel. This gets you about 720m/s, and about 250m frag range. Decent for a large SMG. This cartridge would be too long to feed through a pistol grip, but that's a small trade off. The extremely tight twist an LD6 5.56mm needs is also nice if you would like to shoot subsonics :')

EmericD said:

Why would you want a second caliber for you IW [if you had .264 LICC for LMG] ?

The .264 LICC is probably the best off the shelf GPC right now, and I think it could indeed make for both a fine Carbine round and great LMG round.

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 

VPMudde said:

or the high end I squeezed down a .243 winchester: 6.8 SPC shell shock case necked down to 6mm, loaded with an 81gr ~0.25 G7 EPR to an overal length of 64mm. At a tad over 80kPSI and a 420mm barrel this gives you a 1000m/s MV, 600m fragmentation range, 1200m supersonic range. 

For the low end I was inspired by the AK47 being considered a submachine gun: 5.56mm 74gr ~0.25 G7 EPR sat in a 7.62mm sabot, launched out of a .30 carbine shell shock case loaded to >80kPSI through a 250mm barrel. This gets you about 720m/s, and about 250m frag range. Decent for a large SMG. This cartridge would be too long to feed through a pistol grip, but that's a small trade off. The extremely tight twist an LD6 5.56mm needs is also nice if you would like to shoot subsonics :')

Your 6mm is very similar to what I had in mind for the LMG - so you have impeccable taste ;-)

Your carbine round is awesome, very clever. And I think the ability to also fire subsonics is a cool feature - and potentially very useful is acoustic gunshot detection becomes more of a factor on the battlefield as Emeric mentioned. Biggest issue I see is getting sabots to work with acceptable accuracy.

Your choices (a 6-6.5mm IW and a .264 LICC LMG) seem like they'd be very close one another in terms of weight and performance.

What were you thinking in terms of projectile weight and velocity for the carbine load?

gatnerd said:

Your 6mm is very similar to what I had in mind for the LMG - so you have impeccable taste ;-)

Great minds think alike, no? I think this is what Stan's 6mm Optimum would be with 2020s materials.

Your carbine round is awesome, very clever. And I think the ability to also fire subsonics is a cool feature - and potentially very useful is acoustic gunshot detection becomes more of a factor on the battlefield as Emeric mentioned. Biggest issue I see is getting sabots to work with acceptable accuracy.

Keeping in mind you can buy a 1 MOA capable AR15 these days, 4 MOA from a sabot carbine seems a fair expectation, no? I think that's good enough. 4 MOA at it's frag range of 250m gives you a 25-30cm spread - acceptable (question mark?) for point targets.

It's more of a FISH and CHIPS kind of gun anyway. Out in the hills it's a noisemaker to keep the enemy down until your DM sets up his 6mm Optimum HK416½

gatnerd said:

Your choices (a 6-6.5mm IW and a .264 LICC LMG) seem like they'd be very close one another in terms of weight and performance.

I may be wrong, but I'd think that a 6mm IW round using the LICC case technology would weigh circa 30% less than the .264 LICC.  Increasing IW caliber to 6.25mm or 6.5mm would, of course, reduce the weight difference relative to the LMG round.

As for performance, my intent is for the IW ammo to be as close as feasible to LMG velocity and ballistics.

gatnerd said:

What were you thinking in terms of projectile weight and velocity for the carbine load?

I don't know of any way to determine optimum projectile weight without testing.