I think Franchi's anology to a car spring is not without merit. Both are springs and both made from steel. The part that was not mentioned about the coil springs of a Datsun 240Z was what happened inside the shop where the springs were reworked.
I've had springs from cars and even a couple airplane landing gear legs reworked. In all cases, the springs were re-formed to their original shape by being stretched in the cases for coil springs, or re-arced in the case for leaf springs and the set of airplane cantilever spring gear. A sagged coil spring has been compressed and has taken a shorter set height. It must be stretched back to original shape so it can give the original pressure and height. Simply re-heat treating won't restore coil spring height nor a gun's "hat pin spring" pressure.
