For a first time process you could cast the lower out of melted aluminum beer or soda cans. But its not a high strength aluminum allow. Obviously almost any metal would work that can be melted. And anyone can argue themselves blue which is better. But if it fired at least a mag full its a success. Even if its heavy, chunky and full of voids if it houses the important parts and allows them to work then its a success.
The cat is out of the bag, its pregnant and its going to propagate explosively. I visit firearm forums a lot and its amusing how many internet ego experts are popping up claiming it won’t work, it will be ugly, they who make their own are not of the elite bling crowd, yadda yadda yadda.
I see history here when during the Revolutionary war people were making their own rifles, its time we do it again.
You could cast the parts out of beer cans - absolutely true. The only problem in using that particular alloy of aluminum is that it is strain hardened, not heat treatable. FWIW, you can find the same type of alloy in huge, easy quantities by scarfing it off some aluminum siding from a house tear-down, or gutters in the local dump.
There are other alloys which you could find - like the aluminum used in crackcase housings off diesel engines, the pistons off diesel engines, etc. These alloys all exhibit better strength and casting properties than most of the other stuff you could find.
People would do well to think in terms of “Golly, the local auto wrecking yard is positively FILLED with metal... what *is* that metal in there?”
There’s quite a few very useful alloys that are very commonly available in a junk yard - if you know where to look. Axle shafts, transmission gears, pistons, crankcase castings, wheel castings, etc.