I do not have an FN 509, but I have an FN FNS.
While it has a lot of positive aspects, IMO, it was a few negatives. The worst one is the obnoxious > 7 lbs trigger pull. I guess the lawyers in Belgium have even more power than in Austria.
I could shoot the gun, but it is painfully annoying to wrestle that trigger. I was on the brink to go and trade it in an LGS.
After watching several videos on how to disassemble the FNS and the 509, I concluded the relevant for this thread internals are the same or similar. So I decided to risk it and get the M*CARBO 509 kit for my FNS.
The installation was effortless following the videos. Everything fits perfectly. The original sear spring was with an older design - it hooks in a small hole in the sear. However, the new one works just fine. It uses the other side for support.
The results are fantastic; the trigger pull weight dropped by 2.5 lbs.
I believe it can drop even more (say 1/2 lbs) if I polish the internals properly.
When I disassembled the gun, I was amazed by how roughly machined are some of the parts. The striker has a hard time coming out of the channel, and the sear leg was scratching on the sear cage(or whatever its name was). I took out some of the roughness, but I did not spend much time on it, as I was very eager to finish the spring installation.
To be honest, I have not had the chance yet to fire the gun at the range. In the worst possible case, if I am getting light primer strikes, all I need to do is put back the lawyer friendly original striker spring. If that brings the trigger pull weight by a pound, it is still a pound and a half reduction.
The bottom line, this kit is a must for the FNS owners. Probably the same is true for the 509 owners, but I have no first-hand experience of how far the lawyers were able to influence the trigger pull.
To preempt the FN fanboy comments - it takes zero effort to ignore the thread and continue using the stock springs.