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5.5 oz with the sling loop,rivet, and front sight pin/spring/detent.  The sling loop and rivet add .4oz

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I'm working on a KISS carbine project and I'm trying for 5lbs w/o the mag.

5.0 lbs is a little difficult on accurate scales without involving some plastic / carbon fiber / AR-15 carrier/ etc.  5.5 lbs is very doable in aluminum with A-1 or flat top.  Problem with flat top is adding a couple/few ozs in rear sight.  The KAC's are pretty light in comparrison (10.41 oz assembled on FT) but I'm gonna assume in retroland it's either 604 or SP-1 upper.  603 assembled upper with wear comes in at 9.92 oz or 1/2 oz less than flat top and KAC.  Should be very close with BM superlite.  I used a type D and I came out closer to 5.5 lbs with M-14 sling and empty 20 rd mag using 604 and J&T barrel .  GP sling will save a couple ozs. Don't have an assembled 604 or SP-1 to weigh at present.

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I'm using a RRA 9mm Flattop slickside upper modded for 5.56 (upper weighs 7.1oz.) w/DDA1.5 BUIS (BUIS 1.7oz.)

Ouch!  Not what (I'd do with a dedicated 9mm upper but it's yours.  Is that with modded 9mm door?  Would think it would be heavier than standard with big hump on side.  Sounds like good weight savings.  Never thought to try one of those.  It should really look different.  M-16 carrier is a hunk of metal.  Most carbines are happier internally with them riding along.  Everything that says smooth running adds weight unfortunately.  If you get a chance put one of those SP-1 uppers on the scales for me.  Just curious.  Gotta be at least an oz. or lighter lighter than 603 and possibly less than FT.  Don't know but I'm guessing you weighed everything.