I agree with Technosavant and would get midlength gas with a 16" barrel or carbine length gas with 14.5". Both give you near the optimal 7" of barrel from the gas port to muzzle, the same as the original 20" design. A 16" barrel with carbine length gas takes you up to 8.5" which does batter things more over time, though you can add an H2 or H3 buffer to try to tame the recoil.
I bought a stripped DDM4 V3 style upper about two years ago and have no problem with the fixed FSB. I also use the bottom 1/3 cowitness or just flip down the rear sight so the red dot "floats" over the front sight. Either way the FSB doesn't bother me. And it doesn't interfere with a 3x scope.
My upper is the LW profile 16" midlength and it's works out very well for me. Only the stripped upper/BCG and LPK are DD. The rest is a S&W stripped lower, BCM receiver extension/H buffer/Gunfighter Mod 4 charging handle and Magpul MOE handguards, MOE grip, CTR stock and MBUS rear sight. If you're interested, doing it this way kept it right at $1K. The DD upper came from Smartgunner.com, prices today are of course higher. Last time I checked $409 with free shipping but they're out of stock a lot.
Last edited by Quentin2; October 1, 2012 at 04:33 PM.