Anyone ever build one? Pencil bbl is out because of the bore size (I assume), where else can I save weight? I'd like to build a girly AR for the wife. Major comlaints are too heavy and too loud.
I figure cutting the bbl down to 10" and adding an aluminum extension (ala XM177) would save some weight. Any more idea's?

You can actually go pretty far on a 9mm barrel.  A 9mm pistol barrel is usually around .6", and that's the same diameter as an original pencil barrel, give or take.  9mm peak pressure is much lower than 5.56/.223, so the barrel doesn't need to be as thick.

A bigger issue than pressure with a thin-walled, long 9mm barrel would be warp or droop as the barrel heats, due to the thinner barrel wall, which would make your POI wander some.

If you'd rather keep the thick barrel wall, you could have it fluted instead.

It could be done and heat will be less of a problem with then pencil barrels.

mark
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You want ultra light? Make it an SBR. Cut the barrel to 8 inches, no muzzle device. carbon fiber free float tube. A3 receiver with a lightweight optic on it. Use a light stock, your choice. That ought to come in  under 5 pounds, easily.