I would always suggest you follow the type of advice that HiPressure gave you but, the M15 is an all steel Smith and Wesson K frame revolver - it is the Combat Masterpiece - and that frame gave birth to the M19 S&W 357 Magnum and later the M13 357 Magnum, so yes it can handle the +P loads but even the factory literature on the M19 provided for regular 38 Spl use with 357 loads when you wanted them.
Years ago the K frame 357s were fine with the heavier 357 loads, esp the 158 and 200 gn slugs. They did however develop problems with the +P loads in that the flame cutting capability of the faster burning powders would eat away at the spacer washers they then used at the end of the cylinders. Your M15 may have the same construction which would/should limit the use of +P loads.
That being said however the K Frame Smith 38s were fine with even heavy 158 and 200 gn loads as they used slower buring powders to attain higher velocity. This is good if you reload. If not, follow the advice of a gunsmith for the type of +P loads you wish to use in that 38. HTH. Mikey.
