According to the German source, the firing rate is reduced at any propeller speed due to the trigger delay.
Consider the example of a Ki-84 with a machine gun on its nose with a firing rate of 800 RPM.
- Propeller rotation speed: 1,500 revolutions per minute (one revolution every 0.04 seconds)
- Impulse generation: Twice per revolution (one impulse every 0.02 seconds)
- Machine gun maximum rate of fire: 800 rounds per minute
- Machine gun lock time: 0.017 seconds
At time 0, the impulse generator activates the trigger, the sear is released, and the firing pin strikes the primer.
This lock time phase takes 0.017 seconds, during which the propeller rotates 0.425 revolutions, or 153 degrees.
After the bullet is fired, the bolt cycles: the spent casing is ejected, a new round is chambered, and the bolt returns to battery.
This mechanical cycling phase takes 0.075 seconds, bringing the total time for one firing cycle to 0.092 seconds.
During this time, the propeller completes 2.3 revolutions.
Then, 0.008 seconds later, the propeller reaches 2.5 revolutions, another impulse is generated, the trigger is pulled again, and the second round is fired 0.017 seconds later.
This entire sequence repeats continuously. For this reason, a machine gun with a firing rate of 800 rounds per minute can synchronize with 2.5 propeller revolutions and fire at a rate of 600 rounds per minute.
The trigger delay arises from the fact that synchronized aircraft guns operate in a semi-automatic firing mode, and this delay occurs independently of the propeller’s rotational speed. In semi-automatic firearms, the trigger must be pulled and the sear released for each shot, which means that a lock time is introduced with every round fired.
To eliminate this delay, the use of electric primers is highly effective. The Germans invested resources to adopt electric primers for this very reason.
According to US military research, the lock time for machine guns using a firing pin and standard primer can be reduced to as little as 0.008 seconds, but achieving a lock time shorter than this requires the adoption of electric primers.
In the case of the Ki-84, which has a propeller rotating at 1,500 RPM, achieving a synchronized firing rate of 750 rounds per minute would require:
- a full-auto firing rate of 950 RPM if the lock time is 0.017 seconds, or
- a full-auto firing rate of 830 RPM if the lock time is 0.008 seconds.
Some sources state that the Ho-103 and Ho-5 had firing rates exceeding 900 RPM, so it is plausible that they were capable of achieving a synchronized firing rate of 750 RPM.
Japanese 12.7-mm (.50 in.) Aircraft Machine Gun
The gun may be fired full-automatic from the open bolt as well as single-shot or semi-automatic from the closed bolt.
Rate of fire: 983 r.p.m. cyclic
Catalog of enemy ordnance material.
https://dl.ndl.go.jp/en/pid/4009929/1/78
Spoiler
“US Armament in the Air War.” USSBS. August 1945. p. 10
https://pdfcoffee.com/us-armament-in-the-air-war-pdf-free.html
During World War II, the Mauser 20mm cannon mounted on German fighters adopted electric primers, which comprehensively solved the issues related to the firing pin. Thirty years later, electric primers were again adopted for the M61 Vulcan cannon mounted on the F-104 Starfighter.
The time required for the mechanical movement from trigger pull to firing pin impact—though dependent on the design of the firing mechanism—is generally considered to be around 0.008 seconds. The United States strongly advocated that using electric primers, which reduce this time to virtually zero, is essential for increasing the rate of fire. This is cited as the reason for adopting electric primers.
Spoiler
“兵器と技術 Weapons and Technology (285),” Japan Defense Equipment Industries Association, February 1971, p. 57.
