A simple solution would be just use a piston driven upper chambered for a 223 necked up to 6.5. Then you can use the same lowers and magazines. The US military absolutely needs to adopt an AR-10 platform to replace the m-14 for snipers and designated marksmen
I tend to agree on the Stoner based rifles. One thing missing from the discussion is the impact that new sighting systems have had on the battlefield. Our soldiers are far more accurate and lethal today thanks to these sighting systems. This erases the need to carry as much ammunition into battle and that was one of the main reasons we selected the lighter 5.56mm round over the .308 and 30-06.
The 5.56mm is a decent round in the open, but in urban combat the disadvantages of the round in penetrating even light cover have been proven. In open desert and mountains the shorter range has also proven a disadvantage because the average soldier can score hits with the new sights far beyond the effective range of the 5.56mm. The heavier rounds are far more suitable for shooting at far distances. This issue is reviewed continually by men who base these opinions on experience.
We already have the round - the .308. There are already solid rifles and platforms available that use this round.
Why? M-14s are accurate, dependable, and available.
The Stoner design is fragile, has an inaccessible chamber, and if it uses the gas tube, messy.
If we’re going to design a new infantry weapon, start from scratch and ditch Eugene Stoner’s cheapo.
IIRC, DOD has been doing precisely that since Afghanistan - first with the Knights Armament M110 and later the H&K G28 (HK417 variant). It's referred to as the "Semi-Automatic Sniper System".