Bomber Defense Weapons

[hokusai]

A question for the air combat gunnery experts.

I realize that all defense weapons of bombers were MGs. What is the reason cannons were not used for that purpose?

Was high fire rate with smaller caliber proven more effective against attackers? Or was it a problem of weight, recoil, costs?

[Diamondback]

Cannons are low projectile velocity, better against big and slow targets. MG's work better against agile, evasive smaller targets--sort of like how you might use a shotgun for birds and a rifle for deer.

[Flying Helmut]

Cannons were used!

Japanese "Emily" flying boat, German Dornier 217Z, B-29 Superfortress (tail gun); the list goes on and on.

Machine guns were FAR more common; cannons were rare, but they were sometimes used.

I would guess that rate of fire, as a deterrent rather than a "knock-down" capability, and weight of ammunition belts to be carried and stored, but I don't know that for certain.

[zenlizard]

Depends on the era, the bomber in question, what the manufacturing nation had available, and the logistics base, but really...in some combination, all of the above.

Certainly, rate f fire and ammunition capacity are a couple of the big ones. With an unstable firing platform, uneven training of gunnery crew, and a frequently wildly maneuvering target, hitting the target at all is often a numbers game...the more bullets there are, the better the chance of a hit. And if you have limited ammunition, spraying bullets all over the place will soon run you out of ammunition. May as well have saved the weight, and left the gun, the mounts and (if there is no other job), the gunner at home.

[Diamondback]

Depends on the era, the bomber in question, what the manufacturing nation had available, and the logistics base, but really...in some combination, all of the above.

Certainly, rate f fire and ammunition capacity are a couple of the big ones. With an unstable firing platform, uneven training of gunnery crew, and a frequently wildly maneuvering target, hitting the target at all is often a numbers game...the more bullets there are, the better the chance of a hit. And if you have limited ammunition, spraying bullets all over the place will soon run you out of ammunition. May as well have saved the weight, and left the gun, the mounts and (if there is no other job), the gunner at home.

This. ISTR that Japanese Zero pilots preferred to use the MG's as "primary" weapons and then save the cannons for close-in stuff like killshots. Gunner training and gunsight technology both play roles too. P-38s were able to use cannons to good effect mainly because Kelly Johnson designed them with an "aim the plane and where the nose points is where the bullets go" philosophy.

[flash]

..I realize that all defense weapons of bombers were MGs. What is the reason cannons were not used for that purpose?
Was high fire rate with smaller caliber proven more effective against attackers? Or was it a problem of weight, recoil, costs?

Availability, size, weight, RoF, handiness in action, practicality all play a role... have a look at some of the links.
(eg: M2 Browning 38 kg, 750–850 rounds/min; Oerlikon 20mm 68kg, Cyclic: 450 rounds per minute)
It appears from casual reading that, unlike in the game, gunners weren't all that effective, and bombers couldn't defend themselves from a determined fighter attack. If they could fighter escorts would not have been needed but that's more an argument for having gunners at all !

http://users.telenet.be/Emmanuel.Gus...n/fgun-de.html
https://en.wikipedia.org/wiki/Oerlikon_20_mm_cannon
https://en.wikipedia.org/wiki/M2_Browning
https://en.wikipedia.org/wiki/Hispano-Suiza_HS.404