A file called ammo.lua in your lua/darkrp_customthings folder (see here) is available for you to customize everything about DarkRP ammo buying.
There are three ammo boxes in there by default: the pistol, shotgun and rifle ammo.
Here's how you create your own:
ammoType: The name of the ammo that Garry's mod recognizes If you open your SWEP's shared.lua, you can find the ammo name next to SWEP.Primary.Ammo = "AMMO NAME HERE" or SWEP.Secondary.Ammo = "AMMO NAME HERE"
name: The name you want to give to the ammo. This can be anything.
model: The model you want the ammo to have in the F4 menu
price: the price of your ammo in dollars
amountGiven: How much bullets of this ammo is given every time the player buys it
customCheck: (Optional! Advanced!) a Lua function that describes who can buy the ammo. Similar to the custom check function for jobs and shipments Parameters: ply: the player who is trying to buy the ammo
Any/all the ammo can be removed without breaking the gamemode. Just disable them in lua/darkrp_config/disabled_defaults.lua
Example 1: Adding rifle ammo (in by default)
Example 2: Adding rifle ammo for donator/admin only using FAdmin (replace the original rifle ammo with this)
Example 3: Limit the pistol ammo to gun dealers only.