One thing I'd keep in mind is that different springs (design, materials, #windings, gauge, etc) may not necessarily lose strength in a manner predictable by free length. At least, not when it comes to a standard thumbnail reference of how much shorter it may become compared to when a round wire spring is being used. (Or, a pair of round wire springs.)
For example, I've noticed over the years that the flat wound recoil springs used in the Chiefs Special series get a lot shorter, a lot faster, than the longer round wire springs in slightly bigger 3rd gen guns. However, they don't seem to be exhibiting a corresponding loss of tension compared to their loss of "free length" (at least as soon as the round wire springs).
Puzzling, if nothing else. Every time I've asked about it, though, I've been told the replacement recommendation isn't any different than the other 3rd gen guns which use round wire recoil springs.
I've also tended to err on the conservative side of safety when it comes to replacing them (since we're dealing with aluminum alloy frames, and not steel).
Ever notice how the flat wound recoil springs tend to usually have more windings than wire springs? I'm wondering if the loss of "free length" means the same thing as it does with round wire springs.
We're certainly seeing more use of flat wound recoil springs in new guns, and the recommendations for service replacement of them have been all over the board when it comes to asking the major gun companies using those designs.
Then, of course, we have the dual spring systems that are seeing use.
There's really an interesting science to these things.
