Low Carbon steel is so inferior and difficult to work that only about 5.5 million Garand rifles were built out of it. Production was so difficult that by 1943 Springfield Armory was making 3,000 per day.
8620, the material used for the receivers and bolts has 18 to 23 point of Carbon, thus qualifying as low Carbon steel. It's an alloy steel, but low Carbon just the same.
It's the ideal material. Toughness in the heat treated state is at least as important as hardness. Differential hardening has been known for centuries.
High alloy steel with Carbon upwards of 40 points can be used for receivers and frequently is. The point is that it's far from universal even today.
