M4s are shit simple to make ultra-light. Don't worry about the receiver, unless its in characteristically heavy, though I'm not sure which companies still have that issue. There's polymer, thermomold parts available for stocks, grips and sights. You also have the ability to swap out things like barrels and buffer tubes for aluminum ones. Grab a CQB length rifle to shave off some poundage. Search minimalist M4 to get some ideas. Just remember, cut weight intelligently. You want a practical and efficient weapon over as light as possible, but you'll find a happy balance.
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