A friend and I worked out many of the details of how Pistolero works, and I thought I'd share them here in case they're of interest to anybody else (especially since it's one of the directives available in Season 6 for the open world and missions), or anybody has any further comments.

Pistolero is described in-game as "Start with no ammo. Hostiles killed with sidearm are the only ammo source." It's a bit more subtle than that, though.

When you "start" all the ammo is removed from your inventory and your equipped weapons (excepting pistol) only. However, any other weapons in your inventory will keep whatever they had in their mags at the time. So you can, for example, load a Stoner LAMG with 200 rounds, unequip it, and at any time thereafter when you equip it it will still have 200 rounds in the magazine.

"Start" seems to be defined as

  • Entering a mission area from any other area (open world, safe zone, or another mission). It doesn't matter if you enter at the start or fast travel into the middle of a part-way completed mission: if a mission objective appears next to your minimap, your ammo will be removed.
  • Leaving a mission area. Ammo removal occurs as soon as the current mission objective is replaced by the "Leaving Mission Area" notification.
  • Entering the open world from a safe zone, but only if your ammo was restocked. You can enter the safe zone just outside a safe house without harm; if you don't actually go into the safe house your ammo will not be restocked and when you leave the safe zone your ammo will not be removed.

Note that fast travel in and of itself does not affect ammo: you can fast travel to a Control Point or a mission that's not yet been started (which leaves you just outside the actual mission area) and your ammo will not be cleared.

Ammo drops only from pistol (and pistol-slot shotgun) kills (excepting skill proxies), but is not guaranteed to drop. It's highly likely to drop if you have little or no ammo, but once your inventory is around 50-70% full a type the chance of a drop seems to go down, and at the higher end of that the drop may be smaller than normal. However, if you make enough pistol kills without picking up the dropped ammo, you can reach the maximum inventory level for that ammo type. (I have seen this frequently with Rifle ammo; I don't recall if I've ever managed it with any other type.)

Only ammo for your equipped weapons will drop, and each weapon is treated independently: i.e., how much ammo you have for one weapon does not affect drops for the other weapon.

Talents that return or generate ammo perform as usual. Lucky and Boomerang will return certain shots and not affect your inventory, on a kill Chatterbox will generate ammo for the magazine refill (not take it from your inventory), and the Survivalist's Scraping By and Gunner's Supply Line specialization talents will generate ammo as usual (including, for Supply Line, generating ammo for all weapons).