First things first you need to get the magnum from the power supply zone. (You may need to beat the game before being able to get the encounter.) You need to go through the level and going into all the switch rooms as possible. There you will encounter a monster with "???????" as there name if you are luck. If you get the boss fight animation then your good.

After beating it you will get the magnum and the crafting recipe for it's ammo.

The problem is that "Normally" It would take a lot to get magnum ammo as it requires 7 regular pistol ammo. However, the game every time you get back to the safe house will give you 60 pistol ammo no more no less. Since you get the 60 every time regardless and farming is a toll the normal way. The magic is in the battle simulator, every time you go the sim it counts as leaving the safe house. You don't need to fight anything you can just leave the second you get there and go back to the safe house and get the full 60 back.

So all you need to do is craft all the ammo you can> go to the battle simulator> leave the sim> repeat.

The only real limit is the amount of catalyst you have. If you have beaten the game and haven't sold it all then you should be sitting in the ball park of around 100 or so. Either way, this method saves you a lot of cash from buying ammo and doesn't hurt your runs if you want to use the regular pistol. It is even better if you hold onto the pistol ammo and craft more on the way on an adventure since you the magnum as a max ammo limit you can bring with you. (Max being 42 bullets). With that I hope this guide helps you through the game.

Have a good day