as seen in fig2 the body 51 of the muzzle plug 33 provides a housing 58 for the lock cylinder 76 the locking member housing 44 of the chamber plug 32 is hollow and forms a bearing 47 for the shaft guide 72 a circular chamber 48 to rotationally receive the locking member 73 and a diametric slot 49 to slidably pass the locking member 73 into the chamber 48 the block 56 has a longitudinal opening 59 aligned with the opening through the circular tube 55 so that the shaft 71 can extend through and rotate in the block 56 with the square tube 74 rotating in the circular tube 55 the block 56 also has a radial opening 61 extending into the longitudinal opening 59 a pin 62 is threaded into the radial opening 61 with the shaft 71 inserted into the longitudinal opening 59 in the block 56 until the opening 59 aligns with the annular channel 78 in the shaft 71 the pin 62 is screwed into the block 56 until it penetrates into but does not contact the channel 78 thus the linkage 34 is free to rotate in the block 56 of the muzzle plug 33 but cannot be longitudinally withdrawn from the block 56 to install the gun barrel lock 31 in the shotgun barrel 21 the stop to stop distance from approximately the center of the chamber reduction 26 to the choke seat 28 is measured the shaft guide 72 is inserted through.