800 German-made 20 mm Mauser MG 151/20 cannons and ammunition supplies were imported to Japan by submarine. The Hei (1c) was built in conjunction with the Otsu variant on the Kawasaki production lines but some "conversion kits" were directly sent to New Guinea. In this variant, the wing machine guns were replaced by Mauser cannons. The fuselage was longer (engine bulkhead forwarded of 19 cm, added a small fuel tank). Trial fittings found that these could be placed into the existing wing if the gun was laid on its side and a fairing was provided on the underside of the wing for clearance of the breech mechanism. 388 Ki-61s were so modified; the first of these was completed in September 1943 or January 1944. The last was completed in July 1944. Max takeoff weight 3,470 kg, max speed 580 km/h at 5,000 m, time to that height was seven minutes.