I big reason for no slots in the bottom of the flash suppressors is to cut down on the muzzle blasts blowing so much crud when fired in the prone position. You still want to try to keep the area in front of the foxhole as damp as you can (water, urine if you don't have water) to keep the dust down. It can give away your firing position.
With handguns its pretty well known that heavier bullets print higher on the target (at normal handgun ranges) than lighter bullets. The reason stated (and sounds/feels completely plausible) is that the lighter bullets kick less and are gone from the muzzle sooner than the heavy bullets.
With rifles its the opposite. At normal rifle ranges the heavy bullets print lower on the target than the lighter bullets. I have a Browning BAR GII that belonged to my father-in-law. He told me it was the only rifle, out of hundreds he'd fired over the years, that would keep Remington 180 and 220 grain bullets in the same 5" circle at 300 yds. He had no explanation for it, just that it would do it when all the other rifles he'd owned/fired could not.
I've always wondered about the addition/installation of compensators on .223's.
