our little mk108 30mil cannon, the certified one tapper as a little quirk. A barrel length that leaves to be desired and that can be seen on it’s ballistics, punching like an anvil and falling like one too.
so I wonder what was the decision behind such design quirk?
My guess is that it has to do with weight and space savings, enabling fighters to carry one or more of these 30mm cannons as opposed to only being carried by larger attackers.
By the time these cannons were being used on Luftwaffe aircraft, intercepting bombers over Germany was the main mission. For this they evidently thought raw firepower more important than high velocity. They were able to cram four of these Mk108 cannons into the nose of the ME262.
Barrel length and propellant amount also go hand in hand. The Mk108 had a more compact ammunition length due to it’s shorter case, which was in turn packed with less powder. Thus a long barrel is not needed. This way you save weight and space for the ammunition, and weight and space for the weapon itself. The shell’s explosive firepower was still the same.
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Iirc, aside from the dimensions being smaller, and thus being able to be fitted better in aircraft, I think it’s also built like it is due to it then being easier to be made
Also drag and integrity.
Having exposed guns can introduce drag even if weight and space is a non-issue.
Plus exposed guns run the risk of frosting over and jamming at higher altitudes. A lot of fighter aircraft had little tapes/cloth covering their gun bays to both reduce drag and preserve the gun’s integrity.
Example/Reading:
It’s not a german source, but it demonstrates the utility of guns short enough to fit inside gun bays rather than be exposed when unneeded.
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Maybe this was one of the reasons why they made the Me 262 A-1a/U1 prototype, and then never went on to put exposed weaponry on production Me262s.
It mounted 2x integral Mk108, 2x exposed Mk103, 2x exposed Mg151
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Basically it was built for lightness and simplicity - the Mk 103 was too big to fit in the standard fighters, so this was designed to be able to fire a good mine shell from a light gun.
the short cartridge for this (30 x 90 IIRC) precluded needing a longer barrel.
Appart from not really needing a longer barrle, duo to already small ammo, the barrle length was chosen to have an as high as possible fire rate (late war developments with the Mk 108B even to 900 rpm) while having an as small and light gun. A longer barrle meant needing to have a heavyer lock and spring (or using special alloys which was not an option), as such short barrle, short travel time inside the barrle, short recoil, a light spring and a good firerate.
True. It’s basically an enlarged MG FF, which only has a RoF of 520.
The real question would be:
What was first?
- The MK 108?
- The 30x90mm cartridge?
Japan built the 30mm Type 2, an upscaled Type 99-2, which fired a lighter shell at higher velocity, with a longer barrel but a RoF of merely 420 RPM, iirc.
They ditched it for the 30mm Type 5, which was a new development, with a larger shell and higher RoF.
While the MK 108 had slow muzzle velocity, it also fired an even larger 30mm shell at higher velocity.
At the end of the war, Japan built the 20mm Type 99-2 Model 5, which boosted the RoF from merely 500 to 750, by shortening the receiver and installing an additional buffer spring.
At the time the MK 108s velocity was probably seen as an acceptable trade-off for having light weight and high destructive power.
Higher velocity would have increase weight and lowered RoF.
The RoF could have been increased, with some additional development time but for the time the MK 108 with low velocity was practically the best option since it
- Was simple and easy to manufacture.
- Was small enough to replace the MG 151 in the Bf 109.
- Offered increased firepower over the MG 151/20.
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MK108 uses blowback scheme, usually found in handguns which limits barrel length, because gases must leave barrel before bolt opens.
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API Blowback.
Which isn’t really comparable to a hand gun, since the round is already ignited while the slide travels forward.
Barrel length isn’t an issue there.
You still have to time the bolt and cartridge exiting the chamber against the shell leaving the muzzle - obviously I haven’t done the math, but the 30x90 has a very short case, so that might well still be an issue:
All the sites I can find suggest that with API you can have a high RoF or high MV but not both - there is a direct tradeoff between the 2.