The mounts f or aircraft machine guns are divided into two classes - fixed and flexible.

Fixed mounts are attached to the airplane in front of the pilot's cockpit, either on top or underneath the cowling, holding the guns rigid so that they may be synchronized with the engine to shoot between the blades of the propeller.

Fixed mounts vary according to the types of guns and planes for which they are intended, usually consisting of two brackets, one of which allows for lateral and vertical adjustment so that the guns may be lined up with the sights. Fixed guns are mounted singly and in pairs.

Flexible mounts are attached to the airplane above the .gunner's cockpit, and permit of moving the guns through a considerable arc of fire independently of the airplane.


FIXED MOUNTS

Figure 22 shows two Marlin aircraft machine guns mounted on the cowl of a DH-4 plane and fitted with C. C. synchronizing gears.

The front mounting bracket is a bronze castin-a and consists of a base plate curved to fit the cowl with two upright side plates, one of which is threaded to receive the forward gun bolt. The brackets are attached to the cowl by means of four small bolts, and are set 14 inches from center to center.

The rear mount consists of a bronze socket attached to the cowl by means of six bolts. This socket receives a brass post, which may be adjusted for height by a knurled nut located underneath the cowl and clamped by means of an hexagonal nut on the lower end of the post. The top of the post is made in the form of a T head, threaded to receive the rear gun bolt. This allows for lateral adjustment, and the gun may be clamped by means of the small bolt provided.

To adjust the guns, the plane is placed so that the Unit, and the Ring an Post sights may be sighted on a target at a set distance. After the necessary adjustments have been made to the sight brackets, they are clamped in place. The guns are then mounted and a sight is taken trough the bores at the same target, the adjustments being made on the rear bracket. This means that the center lines of the gun converge to meet at a predetermined distance. The guns are now correctly mounted and must be firmly clamped.


BELT BOXES AND SHELL CHUTES

The aluminum belt boxes are located inside the cowl and bolted to it, one underneath each gun. The covers of the boxes are shaped to act as a guide tor the loaded belt being fed into the gun, and are provided with catches tb keep these closed. Between the two belt boxes is installed the aluminum shell and link chute for the left-hand gun and ejects through the bottom of the fuselage. This is also bolted to the cowl and is provided, with a cover which fits over the ejection and belt openings of the gun. The shells and links for the right-hand gun are carried down outside the fuselage by separate chutes attached to the cowl.

Care must be taken to see that the belt box covers do not become bent so as to hinder the feeding of the belt. The catches must be kept in proper order so that there is no possibility of the covers becoming unlatched by the weight of the loaded belt when the machine is upside down.