I had a quick look at my 3 shot Berthier and have a hard time seeing how the spring fastens to the follower. Basically remove the spring and heat it red hot and bend a small amount of curve into it. With the spring red hot, quench in water with 1/4" or so of oil floating on top. That will leave the spring brittle. Make up a tin tray and support the spring on edge in it, sitting on top of two 2" finishing nails and almost cover the spring with old lube oil. Heat the oil and get it burning, and when the oil has burned away, the temper is drawn to spring temper. While drawing the temper in molten lead heated to 680 F will also work, if you have a high temp thermometer, the oil method is very dependable. If all else fails, you can make a new thicker spring out of 3/8 drill rod.
cheers mooncoon