Originally Posted by
sinister
I really, really like the Larue for its "Short" bottom rail and "Skinny" profile. Put a Knight panel on the bottom with ergo ladders on the sides and it feels good, with thermal protection when you get to shooting a bunch of magazines (not the proverbial "Mag dump," but shooting lots of targets). Con going against it is spotty availability during covid craziness.
The Daniel Defense Omega free-float drop-in is a good choice if it's an issue gun and you're not allowed to modify the base weapon (i.e., driving out the sight base pins and replacing the barrel nut and delta ring assembly) under threat of military non-judicial punishment.
If it's your gun and you are required (by unit or department policy) to keep quad-rails then I'd try a BCM QRF. I bought one last month for a full-length M16A4 equivalent for $150 from Primary Arms. It's a little "Fatter" than the Larue but not objectionably so. The SLR Rifleworks and Midwest Industries options are good-to-go as well.