The solution for guns that you want to be lighter, yet have the same strength, is Flash Bainite (a specially, though cheaply, processed steel). See http://nextbigfuture.com/2015/12/super-strong-flash-bainite-steel.html
Make the barrel and all components exposed to high heat out of Flash Bainite steel, and the rest can be made of anything at all. Maybe the 2 processes can even be combined?
I went to school with that guy. We called him "Jiffy Stiff".