I hold to my position that until a quantum leap in firearms technology is acquired, there is absolutely no need nor reason to toss out the AR platform for some similarly “gilded” piece of current technology.
Background: The M4 was procured primarily to enable ground troops to better optimize the length (for mounted troops) weight(for light Infantry etc) and terminal effects (nominal combat ranges less than 300m).
As for the notion that a “bigger” round will provide “more stopping power”, hmmmmm, too much assumption of what “stopping power” is and what it means.
To improve the M4, add back a few inches to the 14.5 inch barrel to keep the on-target velocity of the M855 style round above 2400 f/s, which allows/causes the projectile to yaw nearly immediately on impact, breaking in two, and causing quite bit of “stopping” power.
If you must go for a new cartridge, add some body diameter to the 556 round (at the cost of magazine capacity), add some more, slower, powder (with the added barrel length, maybe a little diameter (6mm maybe?)as long as the striking velocity at expected combat ranges enables the terminal effects to be as good or better than the the M855 or even the M193 rounds at proper velocity).
Remember, adding horsepower to a firearm increases recoil and blast, (if the platform remains unchanged in terms of weight)reducing hit potential and negating any perceived or real increase of “stopping power”. Recall that old adage “ a hit with a 22 is better than a miss with a 44?
The trade of between bigger/faster bullets and weapon weight/length is unavoidable given today's technology and the laws of physics.
Sure, my Ruger No 1 single shot in 416 Rigby has tremendous “stopping power” and fairly light weight (9.5 lbs), but most Soldiers and civilians I know who have tried it, tried it only once. Only one desired to try it again. I actually enjoy the discipline it takes to force myself to carefully press the trigger, knowing that each shot pretty much unleashes a mini-car crash effect (you know, the funny stars you see and the odd taste in your mouth after a car wreck or a good wack to the face?). I guess I am pretty accustomed to it by now, though, I don't experience much of that effect anymore (maybe Muhammad Ali knows what I am talking about?).
Most High power competitors I shoot with think my M1 Garand Match rifle is a “big bore” (30/06) and many will not shoot it as it recoils too much compared to their 16 lb AR15A2 “service rifles” in 223/556. They do however, have a system advantage-less recoil means faster return to position in rapid fire strings, less fatigue and recoil induced stresses, and better ballistics using the latest 75-90 grain .224 bullets and custom hand loads (cheaper to feed as well!).
Want to increase the terminal effects of the M4? Add 3.5 or so inches of barrel, load up a new ball round with a magazine length 70-80 grain ball round with cannelure (w or w/o a steel penetrator) (The SPECOPS family uses a similar round with the 77grn sierra match king bullet), adjust gas port position to accommodate a slower powder and higher extraction pressure dynamics and increased velocity over the stubby M4 tube, and clearly state that this platform will be most lethal out to 350 m, after that, you'll need a larger platform and significant added recoil to generate the same terminal effects.
Remember, there never has been nor ever will be, a free lunch for the Dogs of War.
Other combinations of course will do the same, maybe some what better. The AR platform now is the most developed weapon in history. It is the king of the combat rifles/carbines. Will be for a long time to come.
Let me add: the M4 platform _is_ tempermental about configuration, and all too often people assemble the parts into a sub-optimal system (then wonder what’s wrong). With the right balance of powder, length, twist, weight, composition, distance, etc. the whole thing apparently works very nicely (say: 75gr no-penetrator match ball, cannelured, 1:7 twist, 14.5” barrel, ball powder). Too often the wrong combination is used (say: 65gr steel penetrator, 11.5” barrel, extruded powder) to lessened effect.
The M4 is a _system_: with the right components, it works great; skimp or overgeneralize and it suffers. While some systems inherently work across all “normal” configurations, some don’t, and this must be taken into account with Stoner’s good but cranky design.