Low carbon (1020) steel is rarely used in making firearms. It has very low tensile strength and is used for things like fencing or building material. Converse to your post, it is high carbon steel (1040), or similar, that is used in firearms production and 4140, or similar grade steel, is turned and machined for barrels.
True today, but historically false for most of the 20th century in most of the world. Still false for most of the parts that go into Spanish artisanal shotguns, except for the barrels.
