The problem is that the army can't find a weapon that is so much better than the current M4/M16 that it literally won't spend the billions it would cost to replace. You'd have to not only replace the weapons, but the ammunition, training soldiers , training armorers , changing weapons racks ( these are also expensive ) its not just a clear cut "this weapon will poke 7.62mm holes in bodies"
The m4/m16 is an excellent weapons system, moving the piston above the barrel only leads to extra parts that were never supposed to be there with additional problems that don't currently exist.
The piston fad is silly. The ar-15 design already has a piston, its in the bolt.
The only reason i've seen M16's jam is due to magazines that our supply sgt will not discard, the feed lips spread and will double feed or sometimes triple feed cartridges. I've had filthy dirty weapons, just apply lube and run them wet and they literally keep going. Hell there's a video on youtube of an m4a1 being fired on full auto until the gas tube actually melts and its still able to fire by cycling the charging handle.
The problem with the m16 is this. Training. Incompete soldiers who think they're hitting something when they're not. Combat arms soldiers will tell you the m4 is awesome. Go ahead ask them. REMFs will say it sucks because they're REMFs.