After getting no results from searching for a replacement recoil guide rod and spring for the FNS longslide. I modified the factory recoil spring using a file and plier. It now shoots fine with my 147gr reloads. The Glock flat wire recoil spring has the same specs as the FNS factory spring.
did you buy a Glock spring? From where? How much?
FN only sells the assembly, and it is like $14...
I got the springs from shooters connection.