Abstract: A magazine attachment/detachment mechanism (10) is equipped with; protrusions (11a, 11b) extending downward from a receiver (2); and an attachment/detachment unit (12) that is movably attached to the protrusions (11a, 11b) and that attaches a magazine (3) to the receiver (2) in a detachable manner. The attachment/detachment unit (12) attaches/detaches the magazine (3) to/from the receiver (2) by moving between a securing position that secures the magazine (3) and a release position that releases the magazine (3). Thus, the magazine (3) is attached directly to the receiver (2), so deviation from the design value for the attachment height of the magazine (3) with respect to the receiver (2) can be reduced. Accordingly, it is possible to prevent defects in the chambering of cartridges without reducing the strength of the receiver (2).