Furrer Automatic Aircraft Cannon

The Swiss Air Force in 1933 inirochicerl an aircraft cannon designed by its well known inventor, Col. Adolf Furrer. I bis officer originated a system that employed short recoil for operational energy and a clever method of timing the weapon to fire slightly out of battery while securely locked. This last feature permitted high rates of fire and gave a definite buffing action on the counterrecoil stroke.

The first Furrer aircraft cannon was 20-mm. air cooled, belt fed and short-recoil operated, with a quick-change barrel having an attached bolt assembly. All Furrer mechanisms are highly characteristic of the Swiss genius for precision-made instruments, being composed of a multiplicity of intricate components that perform reliably but do not lend themselves to mass production.

The aircraft models, regardless of caliber, had the following details in common: (1) Feeds interchangeably from left to right and vice versa; (2) possibility of mounting for either fixed or flexible; (3) a built in rounds counter to give the gunner an instant check on ammunition supply; (4) feed pawl disengagement for bring-inĀ« the bolt home on an empty chamber to prevent cook-off; (5) a non-disintegrating metal belt that did not separate when the cartridge was pushed out; (6) muzzle booster and front barrel bearing; (7) considerably larger barrels than usually employed with the same mechanism in ground work; and (8) single grips in place of the conventional two-grip (or spade) arrangements.

The first models made for the Swiss Air Force were designed for firing through the hollow pro peller hub in a Hispano-Suiza engine at the rate of 400 shots a minute. It is of unusual interest to note that the official armament of the Swiss service was all designed by Furrer and government produced. The small arms manufacturing arsenals near Berne were committed to produce every automatic weapon used by all branches of the Swiss service, despite the fact that two of the leading commercial types of automatic cannon (Hispano Suiza and Oerlikon) were also manufactured within the borders of Switzerland. This shows, if nothing else, that this government had unlimited faith in the Furrer action and exploited its possibilities to the fullest.

After the engine-mounted 20-mm gun came a 34-mm version with an identical action that was looked upon with great favor because of the larger explosive charge in the projectile. The muzzle velocity of 3,445 feet per second coupled with a cyclic rate of 350 rounds per minute made it a very formidable aircraft and antiaircraft weapon. The cartridge case used in both guns was of the type known as rimmed and was fed into the weapon by a metallic open-type link. The 34 mm was not only adapted to wing mounting but to turret installations, the synchronization of the weapon making it one of the few automatic cannon that lent itself easily to turret mounting.

The cycle of operation of all Furrer aircraft cannon is the same, regardless of caliber. After the belt, or magazine, is put into place, bringing the cartridge in position to be picked up by the bolt face, the action is completely retracted. When released, the compressed driving spring gives the firing mechanism a thrust forward. As the bolt face comes abreast of the rear of the feeding system, a loaded round is shoved forward into the chamber. On the last fraction of an inch of forward travel the toggle joint is forced into line and locks, cocking the piece. The weapon is now loaded, ready to fire.

The sear is rotatable in the breech-bolt frame, and upon being actuated, pivots, releasing the firing pin to fly forward under tension of its spring and strike the primer of the cartridge. This in turn fires the charge. For the first fraction of an inch of recoil the barrel is rigidly

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