A modular rear sight system, back plate receptacle, and interchangeable sight devices for guns are disclosed. The modular sight system includes a replacement back plate receptacle for guns having a slide with a rear sight and rear back plate section for receiving a cover plate. The back plate receptacle has a mounting base that is compatible with, and fits within, the slide back plate section, and also incorporates an attachment receptor. The attachment receptor receives interchangeable sight members with vertical runway sight features. Interchangeable and fixed sight inserts of various designs enhance optics, visual memory, target acquisition, gun speed, handling and accuracy.