"What will happen if I use bullet less than 120 gr?"
The reason for the wweight limitation is to avoid flash erosion on the face of the titanium cylinder.
Also, this excerpt from the 2006 S&W Owner's Manual
ALL SCANDIUM REVOLVERS FIRING MAGNUM AMMUNITION
WARNING: DO NOT USE MAGNUM® LOADINGS
WITH BULLET WEIGHTS OF LESS THAN 120 GR.
THIS WILL REDUCE THE POSSIBILITY OF
PREMATURE CYLINDER EROSION.
...even the best 9mm is around the bare minimum to meet fbi reqs.
How DARE you suggest a gun owner should read the owner's manual!!!