In this case, the receiver can be made of common 6061 aluminum, the vanilla of aluminum alloys. It’s about 50-65% as hard as a regular steel alloy. Also, the tolerances between the lower receiver and upper receiver tend to be fairly loose due to these things being designed to be mass produced by a large number of manufactures.
The upper receiver and other parts that attach to and go in the lower receiver have tighter tolerances but aren’t regulated and can basically be bought off the shelf.
