Abstract: A heavy support weapon, for example, of 30 mm caliber, designed for firing from conventional machine gun mounts without undue recoil loads transferred to the gun mount. The weapon is designed for first round accuracy whether in the automatic open bolt or semi-automatic closed bolt mode. The weapon 10 includes a barrel 13 joined to a barrel yoke 21 in a receiver 14 aft of a front housing assembly 15. The recoil system includes a main recoil spring 30, a secondary spring 23, a buffer group including rods 60, springs 61 and plate 63. A slide bolt assembly 40 includes a slide 41, a bolt 42, a face 43, bolt face levers 44 and actuating pins 45, all within the receiver 14. The bolt face 43 is driven vertically by the bolt face levers 44 to receive a new round, load it and remove and discharge a spent casing. The slide bolt assembly 40 travels forward and aft during the firing cycle.