Which ammo grain is good for this FNS 9MM?
115gr is the common i guess.
147 gr is the highest I seen.
I know the higher the grain the less impact it would have? Or is it lower the grain the higher the speed / power?

Bullet weight & velocity are all over the place, so don't assume something before you see the specs.

Generally, 115gr ammo shoots a bit high for me, about 2-3" higher than POA. 124gr shoots dead-on for me, so that's what I use. The only time I use 147gr is when I use my suppressor & subsonic ammo.

For carry ammo, *I* use 124gr +P of name brands: Hornady, Federal, etc. For practice ammo, I use PMC or GA Arms in 124gr FMJ.

My .o2

Thanks  BobCole. I'm guessing 147 gr. are GOOD for noise suppression?  
I'm just looking for ammo with lowest recoil that has good stopping power.

Be sure the 147gr IS subsonic, otherwise it won't suppress correctly. I could be wrong, but I don't think all 147gr is in fact subsonic. Someone can correct me if I'm wrong there.

Unless you have some carpal tunnel issues, you shouldn't have any issues with recoil in the 115 or 124gr ammo options. If you need to, stay away from the +P rated ammo as it's higher velocity.

Most experienced shooters will agree that the 147gr ammo by & large has less penetration than the 115 or 124gr options.

Hope this helps.