I’m not sure why this wasn’t in Cataclysm to begin with.
.357 Magnum is one of the most common and popular revolver rounds available, and is often regarded as an excellent self-defense round. Its power against a human body is considerable, and is called the “gold standard” of stopping power and an “extremely reliable one shot stopper”.
Now, a few issues could arise for gameplay, due to the fact that .38 ammo could be loaded into a .357 revolver in reality, but you can’t do that in the game. Another problem is than .357 Magnum shouldn’t just be made another ammo variant for .38; .357 operates at a significantly higher pressure than the .38 round, meaning that trying to shoot .357 Magnum ammo out of a .38 Special revolver could have potentially disastrous results.
For actual in-game performance, damage and recoil should probably be somewhere in between .38 Special and .44 Magnum. That’s just a rough estimate, though.
Two weapons to use .357 Magnum ammo:
Colt Python (revolver)
6 rounds, reloaded one round at a time
A .357 Magnum revolver. It was fairly common with some Southern police forces in the 1950s and 60s, and was incredibly popular for target shooting, being available with various barrel lengths and grip types.
Winchester Model 1894 (rifle)
7 rounds, reloaded one round at a time
One of the most popular hunting rifles available, the Winchester 1894 is made in dozens of different calibers and cartridges, and is used for both hunting and Cowboy Action Shooting. This particular model is chambered for .357 Magnum.
I’d make this all myself, but I know very little in the way of C++ programming.