Wwii Nose

WWII aircraft: How did machine guns/cannon fire through the airscrew?

Given that WWII fighters were equipped with fairly standard blowback operated machine guns and cannon, what was the mechanism by which the shells avoided the propellor blades? I suppose this question applies mostly to Luftwaffe aircraft which had nose-mounted weaponry. A subsiduary question might be, why did allied aircraft always carry ordnance that deliberately avoided the issue? Sorry, but I'm just interested.
If the gun were interrupted by the passage of the propellor blade, the sequence of fire would also be interrupted and thus the automatic nature of fire (that the firing of one shell initiates the firing of the next) would be halted. I'm not trying to be funny, I just want to know how it works.

The problem started in WWI. When the air war started, they used to take pop shots at each other with hand guns. the first attempts to fire forward actually shot off the propellers.

When war came, the French Morane-Saulnier aircraft were among the first to fly over the trenches; a primitive method of firing a Machine Gun through the arc of the propellor, using bullet-deflecting steel wedges on the back of the propellor, was installed on Type L aircraft and later on the Type N. A revised Type N made its appearance in the summer of 1915, with the 80 hp Le Rhone engine now standard; it was immediately popular with French pilots but the 25 or so built were scattered among various escadrilles. The British bought another 25 'Bullets' and had moderate success against German observation planes until the introduction of the Fokker E series airplanes.

The German government forced Dutchman Anthony Fokker and Hugo Junkers to work together. This collaboration resulted in some famous early Fokker planes - Fokker gained further infamy with his synchronization gear invention that allowed the machine gun to be fired through the propeller, resulting in an air-superiority briefly known as the Fokker Scourge.

The device simply stopped the machine gum from firing as the propellar passed.

The famous Fokker Dr.I triplane was used by Manfred von Richthofen, the Red Baron, towards the end of his career, who achieved over a quarter of his 80 air combat victories in Fokker aircraft.

Fortunately for the Allies, two new British fighters were already in production which were a technical match for the Fokker, the Royal Aircraft Factory F.E.2b and the Airco D.H.2 ( Geoffrey de Havilland). These were both "pushers" and could fire forwards without gun synchronisation. The F.E.2b reached the front in September 1915, and the D.H.2 in the following February. On the French front, the tiny Nieuport 11, a tractor biplane with a forward firing gun mounted outside the arc of the propeller (on the top wing) also proved more than a match for the German Fighter when it entered service in January 1916. With these new types the Allies re-established air superiority in time for the Battle of the Somme, and the "Fokker Scourge" was over.

Synchronised guns nonetheless quickly became the norm - and later versions of the Nieuport, as well as most new British fighters, were to be fitted with them for the rest of the war.

By WWII the machine guns had developed somewhat, and Britian used 8 machine guns that fired in a pattern the crossed at an intersection, or kill point 500 feet ahead of the plane. The need was for speed, and so they choose petrol engines with multiple propellars.

The main German bomber by comparison was built for long distance, and used diesel, with gun turrets. the fighters like the Me109E-3 which mounted two machine guns in the nose, and two in the wings. Provision for a cannon firing through the spinner was also made in the design, but due to heating and vibration problems, this was never used in action.

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