TheSock: There are a multitude of reasons the U.S. sticks with the basic M16/AR-15 design, mainly revolving around money. It's a 'cheap' weapon system, and it's firmly entrenched in all branches.
There are several designs that are probably better in some circumstances.
The M14 comes to mind, and it's older than the M16.

If such a weapon was completely proven, and somehow able to be able to be funded for procurement, and if the congress would leave it alone, and so on..
It would be perhaps decades before the US eliminated the M16 platform for all the front line troops and Guard units.

I submit the biggest single problem with the AR isn't the gas system, it's the chambering. It was designed as a weapon for airforce security guards, not for precision targeting at 600 yards + as is seen in Afghanistan. I don't know what an ideal chambering would be, but I don't think the 5.56mm is adequate for the conflicts we are encountering anymore.

And 50 year old design..Remember, we're still flying the B-52, and it was supposed to be obsolete and scrapped 30 years ago. Pilots retiring now realize they're flying a plane their dad or grandfather flew. We still fly the C-130 and KC-135, also in similar situations

FYI, 'Tremendous Progress' might simply mean they fixed the problem of melting propellant, which locked up the weapon during chambering, or the primer tape sliding off. Just remember, everything they release is propaganda to help secure their next round of funding. When I see the procurement contract on the Defense Industry Daily or Defenseworld, then I'll believe them.

I also think if such a weapon was to be successfully developed, it would never be allowed for civilian sale, strictly due to the fact there wouldn't be any shell casings left laying around for evidence, current shellcatchers notwithstanding. That would severely limit the market. If the lawyers keep entrenching themselves further into the military to the point it cripples operational effectiveness and are second guessing every little detail, they wouldn't let such a weapon be deployed. Might hinder prosecuting a soldier doing his or oher job.

I could easily see a heavy material, such as brass, being replaced with some form of high temperature plastic, or nito based smokeless powder replaced with a designer plastic explosive propellant. The hardest thing to eliminate in a caseless design is the primer, and that's the rub.
Only logical thing I can think of there is a disposable single use magazine. but even then, shock sensitivity might play a role.