The _piston rod_ is of steel, and is provided with a bronze piston head, screwed against a shoulder at the rear end. The head has three notches cut in its perimeter, which fit over the throttling-bar projections on the cylinder wall. The rear end of the piston is bored out to take the _counter-recoil buffer_. In counter recoil the oil in this bore can escape only by a small clearance. In this way the return of the gun into battery is so eased and regulated that very little shock and consequent derangement of the aim of the piece occur. The front end of the piston-rod is attached to the cradle head, front, by means of the piston-rod nut. The _counter-recoil springs_ (three in number each 36 inches long) are helical, being made from a rectangular steel bar coiled on edge. They are assembled in the cradle, end to end around the cylinder and bear in front against the spring support and in the rear against the cradle head, rear. They are assembled under an initial compression of approximately 750 lbs. which is sufficient to return the gun into battery at the maximum elevation. In place of the single counter-recoil springs a set of three inner and three outer counter-recoil springs is also being issued. The _spring support_ forms a support for the front end of the cylinder and a bearing for the front end of the spring column. It has guide lugs which fit into and glide along guide rails inside the cradle during recoil. The spring support is held in place by the retaining ring. Action of the Mechanism. The action of the recoil mechanism when the gun is fired is as follows:—The gun moves to the rear 45 inches on the cradle, carrying with it the cylinder and compressing the recoil springs. The piston rod being attached to a fixed part of the carriage in front, (the cradle-head) does not move. Therefore, since the cylinder moves to the rear, the oil in it must pass from one side of the piston head to the other. The energy of recoil of the gun is therefore absorbed by the resistance which the oil offers when being forced through small openings between the notches in the piston head and the throttling bars along the inside of the cylinder and also by the resistance of the counter-recoil springs to additional compression. The energy stored up by the springs during this compression, returns the gun and cylinder to the firing or original position. This return movement is eased and regulated by the counter-recoil buffer. The piston rod pull and the spring resistance are transmitted to the carriage, but owing to the latter’s weight and the resistance opposed to the trail spade by its engagement in the ground the carriage remains stationary. Weights and Dimensions. Weight of gun and carriage complete 2,520 lbs. Width of track 60 inches Length of recoil on carriage 45 inches Amount of traverse of gun on carriage 140 mils Nomenclature of important parts of the Gun Carriage:—