I have a new Beretta recoil spring for a 391. Here are the measurments of mine.
Relaxed length: 17-1/4"
Spring diameter: .5"
Wire diameter is .0455"
Six coils per inch
FWIW, my 12 gauge has far more than 10,000 through it and works fine. My 20 gauge is around 2,500 and also works...