Crappy steel.
“Crappy steel.”
Cite your sources and testing labs.
Crappy steel.
There are lots of grades of steel. Perfectly fine firearms can be made with mild steel.
The biggest problem is that mild steel may not have the hardness and tensile strength to resist deformation under repeated firing.
It is difficult to achieve the desired heat treatment and hardness unless you can have a uniform source of steel to start with.
These small shop workmen are working with scrap, and that makes it difficult to control the steel used.