An angle-adjustable buffer tube system is a system that enables fluid transitions between a primary scope or sight and a backup offset iron sight without requiring the shooter to remove his or her cheek from the stock. The system includes a buffer tube and a firearm-mounting tube that is able to rotate concentric to the buffer tube. A stock may be mounted to the buffer tube while the firearm-mounting tube may be screwed into the body of a firearm. A lockable flange assembly featuring a flange, a mounting plate, a flange socket, and a plurality of grooves enables the rotation and additionally is able to lock the buffer tube and the firearm-mounting tube in place after the firearm has been rotated to a desired position. An annular retention plate is utilized to ensure that the buffer tube and the firearm-mounting tube do not separate from each other.