I am assuming the pistol and magazines are clean, properly lubricated and function correctly and the parts and installation are correct. Since you have modified the pistol and have new springs you will need to break-in (set the springs) 200-500 rounds. The break-in period will be different than when the pistol was new. Since the pistol functions properly with FMJ ammunition the HP ammo may not be compatible or the springs need to set (wear-in or break-in). Not all ammunition will work in all pistols, common issues are weak-charge or bullet design (truncated cone or flat point). You were concerned about the magazines being loaded for years, this is not a problem as the USP has modern spring steel and was designed for heavy use. Since the magazines are used and have new springs you will need to function test them.