i think i've got that exact upper... 16" flattop with the 3 rail gas block. i got the gas block simply to mount a flip up front sight. i'll probably make a sling mount for one of the side mounts... and may even incorporate a bipod mount in that, too.
i got the flip up front and rear sights from bushmaster and don't have much of a problem keeping rounds within a foot around 300 yards with the iron sights. i did have to drive the front sight post down several turns, but no big deal.
i bought a removable scope mount that i'll probably end up putting a cheap 10x on. i'm coming up with the plans for a shorter scope mount for a red dot type of scope my local shop has. it's cheap-ish and you can select the recticle from a dot, dot in circle, and cross hairs. i'm hoping that will make the 300 yard shooting much tighter. (well, and maybe shooting with less than 30mph wind gusts)
i figured i'd get a sling because it was hard to hang on to my AR when setting up targets, etc. i dunno, that HBAR fluted 16" bbl with normal handguards didn't feel any heavier than the A2s i used in the army way back when. i didn't really want the standard front sight post so as to not look so obviously like an AR.
i just use my ar at the range and sometimes i get treated to more long range shooting. i'm actually a little hesitant about the red dot for home security... i think i'll leave the iron sights setup. less distraction... i won't have something to turn on, guess what setting, do the batteries still work, etc. (unlike the "real" guys/gals who know when the fight happens, train and prepare for it...)
=) chris