I start these questions with some trepidation, since the military application topic generates a fair bit of steam among many of the participants, but these comments are useful for future discussions in other threads and forums.
...a basic tactic of the Taliban was--and is-- to engage us at 450 yards, knowing that we will have to move in to hit him in the immediate term --and catching us in an intervening trap set in defilade...
Let's see, 450 yards is about the same thing as 400 meters. How well documented is this tactic as opposed to 300 meters or 600 meters?
I know that the Mk 262 is developing a good reputation for precision engagements at this and longer ranges in the hands of well-trained designated marksmen. What evidence do we have the the 5.56 M855A1 has filled in this performance gap for the individual rifleman?
...Any lightweight thing we can do to improve standoff response for the individual dogface infantryman in that critical time before heavy weapons, artillery and/or air support can be brought to bear saves good lives and kills bad people...
My job during Vietnam was bringing air and occasionally artillery support to folks on the ground. I know from that experience that getting air and artillery can be a long and frustrating endeavor. In spite of, or because of, the introduction of modern communications and precision ordnance, the time delay hasn't changed all that much.
Getting air or arty is a lot more difficult and time consuming than getting your personal firearm into action. Further, even in today's precision engagement world, moving ordnance in closer than a half-kilometer is fraught with danger, so the more the squad or patrol can bring to the fight for this intermediate distance, the better their chances.
Unfortunately, we have not seen enough discussion about the assets a patrol can and does carry in addition to their rifles. The answer may be in lighter, more effective grenade launchers, lightweight rockets, and so on instead of marginal improvements in individual rifle effectiveness.