A simple way to get more magnum ammo is to sell the weapon to the merchant and buy it back again. Time Stamps Selling the magnum and buying it back: 0:29 Doing this for other weapons: 2:09 Comments about upgrading ammo capacity: 3:18
As long as you don't get the exclusive upgrade, if you sell the weapon, you can get most of the credits you used to upgrade the weapon back from the merchant. If you have already got the exclusive, then it is better to keep the weapon. Another way to get ammo quickly which I did not mention in this video, is that you can put all your other weapons in the typewriter storage, except the weapon you are low on ammo, say the magnum, then run around the map and open as many crates as you can to try and get more ammo. This takes advantage of the games RNG, which usually spawns ammo and items that you need. Unfortunately, this is not a full proof method because of the RNG. Also, there are some item crates which can spawn more ammo than others, so you need to know which ones to open to increase your chance.
really? I upgraded it fully the second I got to chapter 6 or 7 the castle one you just have to do a bit of side quests instead of just doing main missions
