PURPOSE: To control the movement of a gun barrel and a breech ring freely by making variable the viscosity of a magnetic fluid which cooperates with a recoil piston in a recoil apparatus installed to the breech ring or a reciprocating body by means of an electromagnet.

CONSTITUTION: When a recoil piston rod 8 mounted to a breech ring 1 and a recoil piston moves back, the strength of a magnetic field which an electromagnet 25 generates by a control current from a current controller 30 due to narrow flow passages of an oil passage 6a and a valve 21 is preliminarily controlled in conformity with the positions of the breech ring 1 and viscosity resistance of a magnetic fluid 12 is controlled. Due to this, the movement of the recoil piston 7 is braked so that a sun barrel 2 and the breech ring 1 may be moved slowly. When the force of recoil is eliminated and the sun barrel 2 and the breech ring 1 are forced to move back, the breech ring 1 and the gun barrel 2 move forward and return to their original positions. It is, therefore, possible to change the viscosity of the magnetic fluid 12 freely and adjust a recoil motion freely by controlling the electric current to the electromagnet 25 while detecting the position of the breech ring by means of a position detector 32, not based on the definite viscosity of the coil.