Did a test on recoil springs with my M&P9 Pro: With factory spring, empty cases landed 6 feet to the right and back from gun. ISMI 17lb, non-captured spring (using a 5" Pro guide rod) landed 4 feet back and to the right of the gun. Guess the factory spring is slightly lighter than 17lb, probably closer to 15lb. I have a 20lb spring that I can't see installing, as it will probably be too strong.

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NRA R.S.O. & Life Member

YMMV, but in my 40 Pro Series, with 40 or 9mm barrel and factory loads I typically run a 15lb ISMI springs on a Tungsten guide rod, but swap it to 17lbs when I shoot 357sig. The 15lb spring does work with the 357sig also, but the slightly heavier spring just takes some of the battering of the gun out of the hot round, at least imo. I also couldn't see running anything higher that 17lb - S&W oem is 16lb for all three major calibers, but uses different slide weights to accomplish this, and they pretty much got it right.