Ok. This Special AR Upper is on sale at Gunbroker.
I would not recommend anyone purchasing it, even though it's the strongest, lightest one you can find.
See, it's made of Beryllium, the machining and processing of which liberates Beryllium Oxide, which in turn causes chemical pneumonitis and Berylliosis, as well as being very carcinogenic. Frankly, I think it's safer to work with pure cadmium.
I'll be sending an email to that seller shortly.