Hi Squire,
>I actually think the real end to the .50 came with the Jet Age, where time on target issues began to make things more difficult, ala MiG-15 vs F-86, and the adoption of "cannon only" really came into being post WW2.
Well, "the end" really came in the Korean war, but only because the USAF didn't understand the message in WW2. They were pretty disappointed by their 12.7 mm machine guns in Korea though they were using a version with considerably increased rate of fire - I'm sure you are aware of Project GUNVAL which was initiated to rush the 20 mm cannon into service quickly.
>As far as the ammo issue goes, I also think that is a relevent partial defence of the 12.7mm, you did tend to get almost twice the ammunition as a 20mm in many fighters, but that only offsets it somewhat.
I'm afraid it only looks like that as long as the weight penalty is accepted. If you look at batteries of equal weight, the heavy machine gun actually has the poorest endurance:
1x MK 108 - 634 rpg - 431 kg - 295% firepower - duration of fire: 63 s
1x MK 103 - 314 rpg - 430 kg - 240% firepower - duration of fire: 44 s
2x MG 151/20 - 811 rpg - 431 kg - 149% firepower - duration of fire: 67 s
2x Hispano V - 705 rpg - 431 kg - 145% firepower - duration of fire: 58 s
2x Hispano II - 673 rpg - 431 kg - 125% firepower - duration of fire: 67 s
2x MG-FF - 553 rpg - 430 kg - 92% firepower - duration of fire: 69 s
4x MG 151 - 361 rpg - 431 kg - 103% firepower - duration of fire: 30 s
8x MG 131 - 472 rpg - 431 kg - 99% firepower - duration of fire: 31 s
6x ,50 Browning M2 - 390 rpg - 431 kg - 100% firepower - duration of fire: 30 s
12x Browning ,303 - 863 rpg - 431 kg - 62% firepower - duration of fire: 43 s
In other words, if a 20 mm battery had only half the endurance of a machine gun battery, this was the result of a design choice that either preferred light weight over endurace or (more likely if we're talking about the 4 x 20 mm installations) firepower over endurance.
I don't know if you have seen the chart with the burst limits for the 12.7 mm machine gun that had to be observed to avoid overheating and "cook-offs". You could fire a 15 burst round, than pause 30 s, fire the next 15 burst round, and so on - and after 10 secondary bursts fired that way, the barrel would be so hot that you'd have to stop anyway (165 rounds in five minutes).
If the pauses were extended to 60 s, you could start with a 75 round burst, then fire 10 secondary bursts of 15 rounds each. (225 rounds in 11 minutes).
That doesn't look to me as if 390 rounds per gun as outlined above were likely to be expended. I believe Tony has posted once about the British Operations Research people tracking how many rounds of ammunition the bomber gunners were bringing back after an engagement with Luftwaffe night fighters - it would be interesting to know if similar evaluations were done for fighter guns!
Regards,
Henning (HoHun)