A firearm includes a receiver, barrel coupled to the receiver, and buttstock. An openable/closeable hinge assembly comprising front and rear hinge elements pivotably mounts the buttstock to the receiver. A latch mechanism carried by the hinge assembly includes a latch pin having latch and lock members on opposing ends. The pin is both slidable and rotatable in the hinge assembly. When the buttstock is in an unfolded position in axial alignment with the barrel axis, the latch member engages a latch recess on a first lateral side of the hinge assembly to latch the buttstock in position. The buttstock may be pivoted laterally outwards about the hinge assembly to a folded position offset from the barrel centerline. Rotating the latch member engages the lock member with a lock recess on the hinge assembly to lock the buttstock in the folded position.