It could be argued that gun barrels resist the most forces on firing. I also suspect that classic low carbon receivers were intended to fail slowly, generally bend instead of reaching a sudden fracture point.
Low carbon recivers were used because that was the extent of the Metalurgy at that time in history.
"Sudden fracture point is commonly known as Brittle Fracture and is a condition that a steel can be put into if not heat treated properly known as Martensite.
Recievers were annealed to remove the Martensite structure.
Iron along with its alloying elements form the many phases that steel can go through due to its manufacturing. These are referred to as Phase Diagrams.-
http://web.utk.edu/~prack/MSE%20300/FeC.pdf
The following is a good treatise. Note the section about Martensite.
-Dick
http://web.utk.edu/~prack/MSE%20300/FeC.pdf