Dear Auntie Gun Nut,
I think that the British Bren gun of WWII fame is so cool! Why can't I find a cheap modern semiauto copy?
The Bren gun had a receiver machined from a single forging. The stripped receiver weighed four and a half pounds. The forging from which it was finish-machined weighed twenty-two pounds. That's a big pile of metal chips, even for a CNC machine...