The issue with the replacement calibers that they've developed recently (6.5 Grendel, 6.8 SPC, .300 Blk) are that they are compromise cartridge designs.
They are a compromise because while they are better in terms of terminal ballistic performance than the 5.56mm NATO rounds, (SS-109/C-77/M-855) they are limited by the fact that they are still designed to fit within the same magazine well as the 5.56mm NATO round.
Swapping an upper, bolt, and modifying the magazine a bit while keeping the whole lower receiver means that you're not technically replacing the weapon, you're still using an M-16 FOW platform, and just adding other caliber accessories to it.
Benefits include maintaining the supply chain, the training programs, etc.
Also includes not having to sell the project as a small arms replacement project and get approval. You're just adding a caliber accessory kit to the platform that you already have in service.
If there is to be a new round, then it should not be limited by the design constraint of the dimensions of a rifle's magazine well.
Strangely, some of the rounds that seem to be providing the best split between terminal ballistics, trajectory, and recoil control are REMARKABLY SIMILAR to the .280 EM2 rounds that the UK developed in the 50's. (And then had to ditch to adopt the T65 cartridge that became the 7.62 NATO standard.)
So, if you're changing rounds, and changing platforms, you have to consider the ultimate amount of time/trouble/re-training that will follow.
I like the idea of Case Telescoped ammo, the benefits seem to be there, but the overall cost of switching to it, vs the amount of capability increase that it gives is probably not worth it, unless there was some sort of generational increase in capability, which I don't see.
Would it make sense to start with the MG's first? I think so. How about a look at the CT ammo LMG vs the C-9? Here's what one source has to say about some of it:
In September 2011, 19 soldiers participated in a two-week assessment of the LSAT light machine gun at Fort Benning, Georgia to demonstrate its capabilities against the M249 SAW. In one test the soldiers, half armed with SAWs and half with LSATs, marched six miles in full combat gear then fired at targets to measure stress and muscle fatigue. Another test had the soldiers sprint 200 yards wearing body armor and a basic load of ammunition, then rapidly engage close-range targets. A third week involved soldiers of the 75th Ranger Regiment performing a squad maneuver live-fire exercise in an urban setting. Feedback from participants favored the LSAT for its lighter weight and decreased recoil. Soldiers remarked the LSAT had better accuracy than the M249. The semi-automatic option made it more viable for room clearing. One Ranger even said the LSAT performed better than the Mk 46 machine gun used by special operations forces.[10] 15 out of 19 soldiers that participated in the assessment said they would prefer using the LSAT in combat rather than the SAW. The LSAT LMG is 41 percent lighter than the 21.5 lb (9.8 kg) SAW and its ammunition volume is 12 percent less, enabling all the soldiers that maneuvered the woodland obstacle course to complete it faster when carrying it. Participants also took less time to zero their machine guns when using the LSAT; one soldier failed repeatedly to accurately zero the SAW but successfully zeroed the LSAT on the first try. The LMG users completed the course, on average, one minute and 11 seconds faster than SAW users due to increased mobility given by its shorter length, adjustable stock, and lighter ammo. When firing, gunners felt virtually no recoil from the LSAT LMG.[11] The eight prototype weapons fired a combined 25,000 rounds, moving its cased telescoped ammunition to technology readiness level 7.
Saving that much weight on the LMG would be a very good idea, in my opinion.
Introducing the CT'd ammo for the LMG's would also be a good spot to start. The LMG is already a separate training stream from the rifle, so it would probably be 'easier' to swap it out in the system for training purposes. To the bad, you'd lose the commonality between the C-9 and C-7 for ammo, but the LSAT would be lighter enabling more ammo to be carried...not sure where the break-even point is on that trade-off.
NS