Check out 20 mm Oerlikon Gun pt 2 3D model on CGTrader for $6. High-quality mesh by tiger-fire. Textured.
STL (Stereolithography, filesize: 111 MB), OBJ (OBJ, filesize: 127 MB), 3MF (3D Manufacturing File, filesize: 30.1 MB), FBX (Autodesk FBX, filesize: 73.6 MB), MAX (Autodesk 3ds Max, filesize: 122 MB)