I installed a heavy tungsten rod/spring in my G40 because in intended to shoot only stout hard cast 10mm loads. So far no problems from 220gr HC down to 115gr Defenders. The glock store doesn't the list the spring tension weight only the weight of the unit. Never shot it with the stock rod/spring, but I shoot hard cast ammo through the stock barrel and a 7" KKM Precision barrel. No issues. Hope you figure it out.
Chrono results from this morning. ProChrono @ 10", 5 shot avg. 83 deg F.
Glock G40 10mm stock 6" barrel.
Underwood 10mm 200gr HC FP = 1224fps, 665ft.lbs (Advertised 1250fps)
Underwood 10mm 220gr HC FP = 1191fps, 693ft.lbs (Advertised 1200fps)
Glock G40 10mm w/ 7" (seven)KKM Precision barrel.
Underwood 10mm 200gr HC FP = 1253fps, 697ft.lbs
Underwood 10mm 220gr HC FP = 1248fps, 761ft.lbs