I built that gun a few years back, as I recall, the lightest parts I got was the Colt bolt carrier. It is lighter than all the others because of its semi circle shape at the rear of the carrier. I also used a Colt 4 position stock, it is apparently SLIGHTLY less weight than all the others. I have no idea how. A1 upper is lighter than A2 upper for the lack of elevation adjustments. I also got one without a brass deflector. I could have gotten one without the F/A to save even more weight, but I think its good to have one. I got a Colt lightweight barrel without the bayolug to save a little more weight over the one with the bayo lug. The HiPerform handgaurd saves a noticable amount of weight. Its alumimum barrel nut replaces the much heavier steel barrel nut and the carbon fiber material makes it probably among the lightest of the handgaurd options there is. I punched out the front sing swivel. I think thats about it.
Alone, a lot of these things probably wont make much of a difference. When you add them all together, I think it made it noticably lighter. Even with the Surefire and ACOG, it is probably only about as heavy as one of the other stock out of the box rifles pictured here. The Comp ACOG is lighter than all the good dot sights (EoTech, Aimpoint, Reflex etc) and I am about to get a ScoutLight from Surefire which is lighter then the classic 660 series which is now on the gun.