Run the bsp. Go through one doorway to make the crates yield pistol ammo. Go through the other doorway to make the crates yield 357 ammo.
Each doorway trigger sets a different master item_dynamic_resupply. When a crate is broken, it yields the type of ammo determined by the current master.
You can break a crate or two, go through the other doorway and break more crates to get different ammo.
In-game, you can force item_crates to yield the type of ammo which the player most likely needs. E.g., at the beginning of a level, a player with only a pistol will need pistol ammo, not AR ammo. Later on, after obtaining an AR, AR ammo will be more useful.
Set triggers at appropriate places in your map to set an appropriate item_dynamic_resupply to be master.