Contrary to the "well established internet fact" that 125 grain loads wear forcing cones faster, IMO, the answer to your question is no bullet weights are safe. However, I watched a gentleman wear out two model 19 forcing cones and it took so many rounds that I am not the least bit worried about wearing out my K frame .357s. Both of his forcing cones eroded away so bad before cracking that you couldn't help seeing that they were thinner at a glance. He fired almost exclusively 158 grain jacketed bullets with full loads of 296 or H110. IMO again, if you don't even shoot enough magnums to erode the tool marks out of your forcing cone, you have nothing to worry about. Incidentally, S&W replaced both of my friend's barrels for free, which I've read they no longer have the barrels to do.