Show me some proof on the dust thing and I might believe it, otherwise it sounds like something somebody made up.
hmm i suspect that you've NOT fired an "earlier than A2" M-16, prone, on a dry day..... all it's take is once. Been there, done that (though in my case the rilfe(s) in question were ARs belonging to a friend, that happened to have the 3-prong and A1 style FSes, NOT an M16), went "well THAT would suck if someone was trying to kill you..!"
ok, since i do not at this time HAVE the documents needed to be able to quote chapter and verse on this..... any of you guys out there that have copies of the documents the group that "designed" the A2 produced, or alternately an account from an accepted authority on the evolution of this rifle, detailing EXACTLY why the bottom slots were deleted, wanna cover that part of what he's asking for......
Houndawg..... your DI was Full of Bull droppings!!! (and/or-additionally just pumping sunshine up the tails of all those little pieces of impressionable "Fresh meat" that were under his care, for some reason)
the "compensator" thing is a result of the "rumor/scuttlebutt mill" getting a hold of a rather minor factoid about the "new superdandy M-16A2" (the A2 is what you get when competitive shooters design a combat arm by commitee, a frankenstein that solves NONE of the A1's problems while gutting many it's strengths) and then blowing the supposed effect WAY out of proportion...
the A2 birdcage never was and never has been a compensator, it does have a mild (read as "so small as to be laughable, and VERY neglibile/unnoticable to the shooter") side effect of putting a downward push on the muzzle. the bottom of the cage was closed off b/c the dust signature of the M-16 when fired from a prone postion WAS a major drawback to the 3-prong and A1-style birdcage, flash supressors.
summary.... IF the A2 birdcage was designed as a compensator (it wasn't,) then it is THE biggest failure of the USMC Re-design process that spawned it, b/c it simply does not perform that task to a level that is in anyway meaning ful much less of greater utility to the men/women(now) wielding it.
Edited to remove rather useless and uneeded ramblings and blather. resultant from writig this at 2Am