A breast biopsy lateral arm system includes a removable gun mount which attaches to a carriage that traverses along an X-axis defined by a lateral arm in order to position a biopsy needle relative to a patient. The carriage rides along the lateral arm on self-adjusting rollers which are loaded against the lateral arm by spring members. A cam-actuated carriage slide lock can be used to secure the carriage in a desired position relative to the lateral arm. A cam-actuated removable gun mount lock allows the gun mount to be quickly changed and offset orthogonally with respect to the X-axis. An X-axis stop can be used to establish a position to which the carriage can be returned with accuracy.
