I agree with 2-p and will add a point. The modulus of steel is fairly constant. That is to say, a piece of soft, low carbon steel and an equal piece of hard spring steel will stretch/deflect the same amount for a given load. If the load exceeds the elastic limit of the soft steel, it will be permanently deformed (the piece ruined). However, the hard steel will continue to stretch/deflect under increasing load until it exceeds its much higher elastic limit.
In the case of a thin case of hard steel, we could have the soft steel core yielded before the hard case. Would the hard case increase the load needed to ruin the soft core? Very, very little, if indeed, any at all.
The case is for wear resistance. It was not uncommon for guns to be sent out soft and fired for a season before engraving and hardening.
DDA
