I run a 13 lb recoil spring in both the 9 mm and .40 TS. But I shoot minor in .40. I did find there was more muzzle dip than the 11 lb spring, but it was more reliable so I settled on the 13 lb. If the guns ran reliably with the 11 lb springs I would have preferred 11 lb. But that's how my guns run, and obviously some run fine with 10 or 11 lb recoil springs. I think it's well worth trying different springs to see what works and what you prefer.