I was working on a new rifle design before I moved (what can I say, I have weird hobbies). It is a super lightweight weapons platform, modular like the XM8, able to go from PDW to Sub-machinegun to carbine, rifle, DMR rifle and SAW. It uses a roller-locked delayed blowback like the CETME/G-3 with a recoil reduction system. Chambered in 6.5 Grendel it would use 90gr flat-based bullets for CQB and standard use, 123gr VLD bullets for longer range use and 144gr VLD for special applications. I would weigh less than an AR, be as reliable as an AK and it would be super accurate given good ammo. It would (IMHO) be the Uberweapon until somebody can out with Phasers or phased-plasma weapons, but of course the problem is getting it built, getting it tested and getting it adopted.
Governments do not buy one or two weapons, they by thousands, even millions of them, plus spare-parts, magazines and cleaning kits, tens of Billions of rounds of ammo and all the other things it needs to equip hundreds of thousands of troops, to say nothing of having to retrain all the troops. Replacing the M-16/M-4's would take billions of dollars, which is why they don't want to do it. Even just going to a new upper like the H&K 416 would be massively expensive. No, I fear that we are stuck with what we have until those Phasers come out.