|To avoid derailing any other threads, and because I'm realizing that Dogtag and I may have more in common concept wise than I'd thought previously... Here is my version of his concept:
The background for this idea is projecting a decade in the future, and assuming that a OCSW 25mm light cannon/grenade machinegun is up and running as a heavy support weapon, and that our primary infantry weapons are the LSAT family firing telescoping rounds, either composite cased or caseless. At present this family of small arms is envisioned as using the the 5.56mm round, and some pictures suggest that perhaps an elongated, high BC version with significantly better performance at range is being considered.
But what if instead the weapon used the 129grain bullet and powder charge from the 6.5mm Grendel for similar recoil at a lower muzzle velocity compared to the 5.56mm, but better long range performance due to the heavier round--just a bit lighter than the 7.62mm NATO, and with at least the long range performance of that round? Since entirely new weapons and ammunition are being envisioned anyway, there is no logistic penalty to the move and the weight difference would be minor. The 5.56mm ammunition is doing significantly better in the weight reduction department than the program requirements anyway.
The SAW version of that weapon family would then have performance similar to the M240 in a much lighter and more mobile package, and with extremely light weight ammunition. What would be the logical replacement for the M240? A medium machingun using a larger caliber round such as 8.6mm--probably in either a round similar to the LSAT: either cased telescoping or caseless? Maybe so.
Another option would be a weapon using the 25mm round that is intermediate in weight and firepower between the semi-automatic XM-25 and the Fully automatic OCSW. Perhaps using selective fire between semiautomatic and burst? While firing fewer rounds than M240, a burst of airbursting rounts would be quite effective at suppressing area targets, and single shots would give good capability against point targets including those in defilade.