The bottlecap mine is a powerful improvised weapon. It functions like other proximity mines, but takes the player slightly more time to arm and drop. It also has a longer warning sound before detonating. Like other placed explosives, it cannot be used in V.A.T.S.
¹ Craftable without a workbench via a dialogue option with Veronica, or with ED-E once the add-on Lonesome Road is installed.
Ranger Station Charlie - Enemy-placed mine; must be disarmed. Located in the room to the left of the entrance behind a pair of fire extinguishers. Only appears after speaking to Ranger Andy about investigating why he has lost contact with the station, or starting Return to Sender.
When talking to Alice McLafferty during Pressing Matters, she mentions that some idiots use caps inside explosives. This is a direct reference to the bottlecap mine.
Compared to Fallout 3, the usefulness of the Bottlecap Mine is dubious. Of the ingredients needed to craft it, the Sensor Module is comparatively rare. The Satchel Charge also requires a Sensor Module, but requires a lower Explosives to make, does more damage, takes less time to place, and triggers from proximity much faster.
Alternatively, the Bottlecap Mine does not require Scrap Electronics which, for someone crafting extensively with Mad Bomber-related items, will be in surprisingly short supply. In this case, the Bottlecap Mine provides an alternative to a craftable mine.