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I think if you were to cut key mod into the places where the rail sections were you probably would get about 3-5oz of weight savings. I am basing this on the weight savings on the Vltor Vis uppers with keymod vs standard Vltor vis uppers. 3-5 oz is definitely significant and I would consider getting back into the XCR game if this was an option.
That wouldnt take off very much weight at all, personally I dont like it but its cool that you did something different. You shouldnt have shown everyone though now they will all be doing it for "huge weight savings" lol.