[TooSixy-closed]

OK. So your USPc was a V3 before you switched. Did you mean to say V1, because V3 is a DA/SA with a decocker but no safety. Is that what you had?
The short hammer spring in the LEM kit is meant for the HK45c. The longer one is the one for your USPc. What trigger did you switch out? Did you mean the trigger rebound spring (TRS)?

If I understand you correctly, you're now using the hammer spring for the HK45c and the original V1 Trigger Rebound Spring?

 

[TooSixy-closed]

Did you purchase the gun new or used? Was there any custom work done on that gun? It sounds as if the original hammer spring was modified (shortened) or replaced with a lighter one as Big Bore pointed out.

I have a couple of V1 USPc's and the only difference between my V1 and your V3 is the safety. I have Light LEM in all of USPc's (original TRS and hammer spring) and they work without any issues.