How to improve the AR-15/M16? Look at the Daewoo, or ZM Weapons approach. AK-style gas piston, but still an AR lower. No more "defecating where it eats", how about that? Betcha the action fouling and subsequent reliability issues would go bye-bye, too.
My 14 years of active duty experience are Air Force, mostly as aircrew, so I can't relate anything about dragging M16's through the mud, sorry. I can, however, relate that the day I qualified with the M16 in basic, back in April of 1986, the recruit to the left of me (a southpaw from the U.S. Virgin Islands, I'll never forget his enthusiasm for the Air Force) had a slamfire that burned his right cornea. He never finished basic.
I've requalified with the M16 several times since then, prior to overseas assignments of for deployments. While I feel the current system of 25yard reduced targets, simulating 200 meter engagements, is weak at best, it is after all the Air Force, arguably the most 9-to-5, Monday-through-Friday branch of the military, and it's a miracle we even shoot any small arms or attempt proficiency. Those of us who do shoot off-duty, seldom have trouble qualifying expert when the time comes. But I sure as heck would rather rely on a Marine or Army member if it got to a ground shooting conflict. The last time I qualified with the M16 at McClellan AFB, my issue rifle was an early variant, no forward assist or brass deflector, vintage bullet-dial rear sight, and as I looked it over, the instructor noticed my confusion. Forward of the receiver, the gun had the 1-7" HBAR profile barrel, round handguard, and solid-bottom flash hider. They couldn't afford complete new guns, so they were just rebarreled. Weird, but it shot well enough to get expert again.
When my wife fired my preban Colt Competition HBAR, the first words out of her mouth were "What's up with the "Sproingg?" She still feels my Kalashnikov is a better rifle, and to this day refers to the Colt as "Sproingg". And I even greased the recoil buffer and spring.
Wonder what John Moses and John Cantius would say if they were around?