I made a spring for an old H & R revolver only last weekend and it worked just fine.
Now I learnt an old gun makers trick years ago, the trick is in the tempering.
What Bill says is spot on but it can sometimes be difficult to get the heat even for the final stage - if the spring is too hard it snaps like a carrot
and if it is too soft it will stay bent and not "spring"
. What I do for the final stage is to place the spring in a shallow pan of motor oil (an old tobbaco tin is good), heat the oil until it starts to burn and keep it burning untill it is all gone but dont play the flame on the spring . You then let it cool down and there you go it is cooked just right.
I understand that they used to use Whale oil but that is obviously not available now.
I have made a few like this have never had a failure.
Best of luck