I have had a Mod 4 for decades now at one point I got cocky and tried a caliber change in the field and lost my spring, stupid I know but I had done it hundreds of times, what could go wrong, right? It is now impossible to find a spring, I keep an eye on Numrich and other sights but no luck yet. Anyone out there have a spring and the means to measure ID, OD, wire size, length and number of coils. With that information I may be able to find a compatible spring, I tried, a few years back, to contact HK and didn't get any response on the spec's for the spring. I bought the gun at Kittery Trading Post many years ago for 195 dollars, if I could find one now for that price I'd buy it and part it out. ;o)} Any information would be greatly appreciated.