I had a 637 for a while, I shot alot of loads through it, +p and not, and the only load that felt "light" was a 148gr wadcutter. You can contact S&W with the serial # and they will tell you the year it was built and if it can withstand +p. After that I think 158 lswchp's work fine. I carried Corbon DPX with 135gr +p Gold Dots as reloads.