The advantages and disadvantages of electromagnetic railguns with rectangular, arc and round bores are compared and analyzed. It is found that the round bore is consistent with that of conventional guns and it is of better inheritance in engineering application. A 30 mm small bore electromagnetic railgun is designed, with simulation test carried out for lumped power parameters, armature deflection force, structure and temperature rise. Current climbing tests are carried out on thin-arm and thick-arm armatures. The thin tail armature presents obvious burning loss at a current amplitude of 265 kA, while the thick tail armature presents obvious burning loss at a current amplitude of 350 kA, indicating that it can withstand 30 stable fires. The maximum line current density achieved by the small-caliber round-bore electromagnetic railgun is 17.5 kA/mm. The firing test at 450 kA will be carried out later. This study can be used as reference to some extent for the application of small-caliber round-bore electromagnetic railgun.