The problem came when heavy shooters and police departments went from the thought process of "carry 357's, practice with 38's", to practice exclusively with what you carry after the Newhall incident. The gun was made for 158 grain rounds using 1950's powders. Bill Jordan also expected the gun to have many more 38's shot through than 357's.

Shooting guns obviously causes the metal to warm up. The flat spot on the K-frame forcing cone (at the 6 o'clock position) could get so hot that it would crack from the heat and pressures delivered by the hotter 125 grain rounds.

I've personally seen several model 19's with cracks in the forcing cone at gun stores. Needless to say, the guys behind the counter were NOT happy when I told them what they had.

A model 19 or 66 will be fine shooting 125 grain 357 rounds in moderation.

EDIT TO ADD: take a close look at the current 357's made by S&W and you'll see a piece of steel on the bottom side of the top strap above the barrel/cylinder gap to keep the heat and gasses from damaging the metal.