here's my .02.  I use a Colt 6920 (effectively a semi-auto M4) and it is great.  I have a friend that has a DPMS carbine and he's spent a good bit more money getting it to shoot like my colt.  He can outshoot me now, but not with the heavier bullets (75 grs+).  He also built a 20" bull barrel flat top from Model 1 parts.  I've handled it, but not shot it.  I have a LE friend who has a RRA standard A2.  I've checked out the Bushmaster, Rem R-15, and Sabre.  All are good, but the devil's in the details.  Colt, Sabre, LMT are about the only ones that are truly mil-spec throughout.  What that means is they are less likely to come apart, which is not really a likely situation for a civilian anyway.  Bottom line, most are good.  For mid-priced rifles, the Bushy and the Rem are great.  The higher end ones (Colt, Sabre, LMT) may last longer, but really won't outshoot the others.  I wouldn't disregard the carbine (short handguard 16" barrel) either.  They are very handy in a vehicle and shoot as well as a 20" barrel.  Unless it's a bench/varmint gun, the heavy barrels make them real muzzle heavy, so I personally don't like them for a defense or hunting rifle in an AR.   The Remington seems to fit your criteria best and as far as I know it's well made and should serve well.