Maybe the table is from a later time period.
The 20mm cannon was more reliable, had a better rate of fire, and more accurate post WW2. The .50 cal guns saw some improvements, but the scope of this discussion should remain in the context of WW2.
I totally agree.
My only argument as for the reliability of the 20mm, is that it must've been faulty enough to prevent the US from wanting to use it on its early to mid war planes, although the British did continue to use it despite some defects and on going development of the gun. It must've been good enough for combat, but not reliable enough for US standards.
US built 20mm HS404 based guns and their ammunition were far less reliable than UK built weapons due to changes that were made by the UK to the original French design. These changes were no made to the US weapons. No one is saying that the UK 20mm was perfectly reliable but it was probably no worse than any other mass produced weapon of a similar type in this period. The thread listed earlier gave examples of the firing tests held in the USA. As for the ammunition the best example I can give is on Malta. They had a huge number of failures and when it was looked into the fault lay with a batch of US ammunition. As a result and despite the desperate shortage of all types of ammunition Malta destroyed all US manufactured 20mm ammo. The problem then went away.
Plus, Britain found itself defending against bombers where larger projectiles would've been preferred. Most mid war US planes were performing escort duties, so there wasn't a need for 20mm, if the .50 proved to be more accurate and reliable.
Britain decided before the war that the LMG wasn't effective enough and decided to go for the 20mm. The 8 x LMG was almost a fall back position which by the standards of 1938 was in itself was more than respectable. These decisions for the UK and USN (amongst others) were taken before the war, the UK going for the 20mm and the US the 0.50 M2. Decisions taken during the war tended to be the result of lessons learnt and/or the aircraft available to the opposing forces. The fact that the UK faced bombers was no doubt a factor but not critical, after all the USN also faced bombers.
For the US forces the 0.5 was more than sufficient for what they wanted it to do and there was no reason for change. Had the US been facing large numbers of B17 or B29 style bombers then its my belief that they would have made changes, but they didn't need to, so why change?
By the late war (1944-1945) the 20mm cannon was the preferred projectile because all of the problems since developing the cannon from pre-ww2, had been minimized. It had an equal rate of fire, and by late war standards, ground targets and their better armor were of increasing concern, which the .50 cal was not very effective.
I find it interesting, because they still used rockets and bombs in those situations, and the 20mm was spared for air to air uses, where .50 cal was as effective.
The armour on ground targets is I think an exagerated point, the 20mm did have better penetration but not enough to damage the majority of targets. It might make a difference if you targeted a light armoured car and got lucky, but not really. If you are attacking a truck then a bullet might knock a hole in it and if it hit a none vital part, do no more, however a 20mm is almost certain to wreck it.
So maybe it did come down to weight and ammo load. But there are pilots who liked having the larger ammo loads of the .50 cal so they could be more liberal with their bursts. I'm sure there just as many reasons for preferring the 20mmm cannon in air to air combat.
If I can break this into two parts.
Endurance of the ammunition load
I agree this is a valid point often ignored. If you are on a long escort mission or at sea where you have to stop the attacking aircraft then how long you ammo lasts is vital. If you are defending with other airfields to divert to then the amount of ammunition you can fire in a burst is probably more important.
I use the example of the Whirlwind. With 4 x 20mm in the nose, for 1940 its firepower was unmatched by any day fighter. But it only had 6 seconds of ammo, say three bursts. If you are attacking a bomber formation when three accurate bursts, each of which may well destroy or seriously damage a bomber, then that is probably fine. On the other hand, would you like to go on a four/five hour mission, hundreds of miles behind the lines with only three bursts of ammo?
Weight of the Ammo / Weapons
This is basically efficiency. In numbers the 0.5 may well be less efficient, but it was more than adaquate and efficient enough for the job in hand.
What is often forgotten is that the 0.5 M2 was a 1930 (ish) design based on a 1919 design. The 20mm Hispano II was a 1940 design (once they made it reliable) based on a 1937 French design. It would be a minor miracle if the 0.5 M2 was as efficient as later 20mm.