Abstract: Due to its advantages of long working length, small aperture, and good rigidity, the endoscope can be employed for the defect detection within the inner walls of small-caliber artillery tubes, thereby achieving the purpose for inspecting and evaluating the quality of the internal chamber. This paper presents the design of a side-viewing dual-optical-path large-depth-of-field endoscope optical system tailored for small-caliber artillery ranging form Ф30 mm to Ф85 mm. By incorporating a shared right-angle prism, the final design achieves a full field of view (FOV) of 68° for the single-optical-path endoscope optical system, with a working distance of 1 099 mm, enabling effective imaging within a depth of field ranging from 12 mm to 40 mm. The effective aperture for light transmission is less than 4.5 mm, exhibiting a maximum field curvature of −0.37 mm and a maximum distortion value of −14.6%. The optical transfer function across the full FOV exceeds 0.2 at a spatial frequency of 45 lp/mm for all configurations, approaching the diffraction limit. The observation range of the dual-optical-path endoscope optical system meets the requirements for parallax generation. The design results demonstrate that this endoscope optical system possesses superior imaging quality, fulfilling the needs for three-dimensional measurement of defects within artillery chambers.