The Ki-44-II "Otsu" (also known as the Ki-44-IIb) could be armed with a Ho-301 40 mm autocannon. While this was a relatively high-caliber weapon, it used caseless ammunition with a low muzzle velocity and short range, which was effective only in close attacks. Some of these aircraft were used against USAAF bombers by a special "Shinten Seiku Tai" (air superiority unit), comprising at least four aircraft, that was part of "47th Sentai", based at Narimasu airfield in Tokyo. Pilots from such units attempted to shoot down B-29s and, once their ammunition was expended, to ram them – effectively a suicide attack. While the concept appeared straightforward, ramming a B-29 at high altitudes was difficult to achieve in practice.