I have a 340PD and it is best to carry and fire 38Spl+P through it. 357 Mag is really to much for a 12 ounce snubbie. It basically doubles it weight when you load it.
When ever I carry it, it is loaded with 38's but I fire a few 357 through it at the range, just to remind me why I carry it with 38's. But besides the horrible recoil from a full magnum load, it is a fine weapon.