What I am saying is what a lot of other people are: the rifle itself being a certain mass for accuracy, weight savings, ease of use, fire control, accuracy control and overall use is just right as originally designed. The best option is to balance any reduction on the rifle itself with what is added on to the soldier as a whole: first look at at the gear, clips, pouches and packs where so many small things matter; then move to the clothing and armor being worn that must be worn or carried; then look at the types of gear like radios and GPS units used that might be fine with aluminum or plastic instead of cast iron (fast example); then look at the ammunition types and amounts carried; and finally look at the additions to the weapon itself and how to keep the overall mass at its intended design amount.
The load on a soldier shoulders and back is not only the weapon: savings abound when spread over everything.
I am also saying that difficulty shooting 556 from a 4lb carbine will be no problem... but doing it accurately without the required night vision and other items will be.
