The issue of 125 gr. bullets damaging the forcing cones was addressed by S&W and found to be without merit. The culprit was not th bullets or velocity of 125 gr. jacketed ammunition but infact was related to a build up in the forcing cone not being cleaned out. S&W says they put 250,000 rounds through a K frame and cleaned it every 100 rounds and nothig falied. But when mixing the rounds with lead, allowing a build up was determined to be the primary cause of fractured forcing cones, especially when a high velocity rounds gets shoved into a dirty forcing cone. Even low velocity rounds being pushed into a dirty forcing cone can quickly cause premature failure, so the key here is keeping it clean.
I've put thousands of 125 gr. jacketed, loaded with H110/296 through my K frames without any problems. In all complete honesty I load exclusively with H110/296 and shoot about 90% 125 gr. jacketed the rest are 110 gr. or 158 gr. jacketed.