The Story So Far
So the likely candidate is a derivative of the Textron LSAT that has been in testing and development for some time. The original prototype was tested in a 5.56 caliber in both caseless and cased telescope versions.
https://youtu.be/qL6pPsEJ6GA
The caseless version, can't really dissipate the heat from the gun like a brass cartridge case can and that was its Achilles Heel. The polymer cased-telescoped round, on the other hand actually reduced the transfer of heat from the cartridge case to the gun, making the gun run cooler than a brass cased round.
The prototype weapons weighed just under 10 pounds empty, around the same as an 8 shot M-1 Garand Rifle. The current M249 SAW weighs just under 18 pounds empty.
Same Dumb Army - Courtney Massengale (Rant)
https://en.wikipedia.org/wiki/Once_an_Eagle
So the same Army that takes decades to select a new handgun and also wanted to go backwards and issue everyone a 7.62x51 rifle, al la the M-14 disaster, now wants to take a prototype proven weapon system and increase the caliber, for no operational reason.
These people don't understand that they are basically starting over, in the weapons development when you up the caliber. Therefore using REMF Logic, order the guns; 100,000 of them, before the prototype is even been evaluated. That way the program has "momentum" and can't be cancelled, like "too big to fail" and the career Army program managers get promoted and moved to better positions before the program fails, etc.
The Real Issue
As of today, nothing under 50 BMG can reliably penetrate Level IV body armor at range. This includes the Russian GOST Class 6a Armor system, being issued today. Level IV armor is designed to survive hits from 30-06 M2 (AP) Armor Piercing Ammunition.
30-06 M2 AP ammunition travels at around 2,800 FPS. The projectile is a manganese-molybdenum penetrating core inside of a copper jacket with lead filler for a total projectile weight of about 165 grains. The core has a hardness of 785 on the Vickers scale.
The 5.56x45 M995 AP Round fired from less than 100 yards will penetrate Level IV body armor. This round has a tungsten core and was developed specifically to penetrate the armor of the Soviet BRDM-2 armored vehicle.
The 7.62x51 M993 AP round was developed at the same time and was also able to penetrate the BRDM-2 vehicle. However, the distance required to ensure armor penetration is not publicly available, but it is undoubtedly "too close for comfort".
Both rounds are usually found in belt fed weapons only and not issued to riflemen.
Strategically, Tungsten is used to make machine tools
only in time of war and top 3 largest producers may not be willing to sell us additional supplies, making its use in ammunition non-viable:
China. Mine production: 79,000 Metric Tons
Vietnam. Mine production: 7,200 Metric Tons
Russia. Mine production: 3,100 Metric Tons
Real Solutions
What rifle you go to war with doesn't determine the outcome of the conflict. Many factors do come into play to create victory or defeat. The side with the best stuff, doesn't always win.
That being said, it is always desirable to have better guns than the enemy. Not only is it good for morale, it might mean that you have to spend less money on body bags, etc. Having a Combat Rifle/SAW system that can reliably penetrate enemy armor and cause causalities is very desirable.
Armor penetration is a factor of velocity, bullet construction. The diameter of a projectile should be the smallest possible for a given bullet weight and velocity.
As of today; the highest rifle velocities achieved commercially, is around 5,000 FPS in competition. Obviously, this is out of a custom bolt action, that likely has low barrel life, but it illustrates what is technically possible.
http://bulletin.accurateshooter.com/2015/08/5000-fps-with-22br-improved-and-30gr-berger-bullets/
As a reference, the 120mm main gun on the Abrams Tank fired a 6 pound, 26mm diameter depleted Uranium Sabot at around 6,000 FPS. The newer rounds have been upped to 18 pounds+ with velocity falling to only (?) 5,100 FPS. Obviously what a 7,300 lb cannon can achieve is going to be different than what can be expected out of a 6 or 7 pound infantry carbine.
Reality Check
Going up in Rifle/SAW caliber is a mistake, as the velocity needed to have equivalent armor penetration will need to be higher, if everything else is equal. Using a SABOT is not practical in a rifle, as the accuracy is seriously degraded. Tanks can get away with using SABOT's as their principal targets are other (really big) tanks.
Although the 6.8 SPC II saw some limited issue, it wasn't widely adopted as it really didn't provide any war winning performance compared to 5.56x45. Although the 6.8 SPC II bested the old Soviet 7.62x39, it wasn't superior to the current issue 5.56 round.
Projectiles that strike the body at 2,200 FPS and higher produce damage to human tissue from the creation of a temporary wound cavity, as well as the permanent wound cavity caused by the path of the projectile. At a striking velocity of 2,200 FPS and above the body is unable to recover from the stretch of the temporary cavity. This damage is significant and can cause permanent disruption to organs, and body tissue not impacted by the projectile.
In contrast, the human body does not take damage from the effects of the temporary cavity produced by pistol rounds, as the body's elasticity is able to recover at these lower velocities.
Based on the 2,200 fps rule, 5.56x45 is able to deliver maximum trauma up to around 300 yards. The 6.8 SPC II falls to 2,200 FPS at around 200 yards, quite a significant difference, especially when you consider that in the past, combat usually occurs at around 250 yards or less, even though general issuance of magnified optics can push this figure out to 400 yards, when terrain allows.
Therefore a 6.8 caliber Rifle/SAW combination is a failure from the get go. Only by increasing the velocity, size, recoil and cartridge weight can a 6.8 diameter round hope to equal the armor penetration capability and wounding potential of a 5.56 diameter rifle bullet.
Another Way?
A path that might yield greater success against armor would be a return to a light 55 grain projectile fired from longer 20" barrels, rather than the current 14.5" M4 barrel. Depending on the ammo fired, the longer barrel will have additional performance against unarmored and armored personal with measurable increases seen from 60 to 200 yards, compared to the shorter barrel M4. This is no small thing.
A new 55 grain bullet, matching the outside dimensions of the M855, but with the penetrator and general construction of the M855A1 may be the way to go. Such a round could generate higher velocity, lower chamber pressure and increased terminal performance and be able to be retrofitted to the existing stock of weapons cheaply, until truly better solutions are available.
The proven belt fed LSAT 5.56 prototype could be fielded with the same improved projectile, in the lighter cased telescoped format. Obviously, the ammunition would not be compatible with the existing service rifles, but that could be solved once a new rifle was fielded to replace the M4/M16 rifles currently in service.