Abstract: A method for locking a firearm in an “open” position involves blocking some portion of the firearm's action (internal operational mechanism) with a selectively-extendable locking device accessible from outside the firearm. More specifically, the locking device is positioned at one of a plurality of pre-selected locations on the firearm, and includes a plunger that lies retracted when the device is unlocked, and that extends into the interior of the firearm when locked. The pre-selected location is chosen such that: (i) when locked, the locking device interferes with the firearm's action, preventing it from being closed, and the firearm from being fired; (ii) the locking device can only be locked when the firearm's action is in an open position, with the chamber visually exposed; and (iii) to the extent one is able to operatively disassemble the firearm when the locking device is locked, the locking device cannot be defeated internally.