The crucial point being that both the .50 and 20mm cross the threshold of minimum necessary power.
The Crucial point is that "the threshold of minimum necessary power" changes with both time and aircraft type.
What specific anecdotes are you referring to? Robert Johnson's experience is not anecdotal, it is documented.
Anecdote or documented, a sample of one (or 5 or 10 selected incidents) is too small to reach a conclusion when hundreds of incidents may have occurred.
I am sure "6 20mm shells hit a plane" on many occasions and were not "devastating" to the airframe and did not result in destruction.
While quite true that is not quite what I wrote is it? 6 exploding 20mm shells may be more "devastating" than 12 .5 in punctures. Neither may be enough to bring down an aircraft. To use a single sample of my own there is the incident of a Short Sunderland that was attacked by eight JU-88s, shot down 4 (6?)of them (using mostly .303s ?) and crash landed after the other 4 left (out of ammo, out of fuel, driven off?) with (it is claimed) 500 bullet and shell hits. The only thing this really proves is that with thousands of air to air combats anything is at least possible, almost the entire crew was lost 2 months later when their Sunderland was attacked by Ju-88s.
"Luftwaffe figures that an average pilot hits a 4 engine bomber with 2% of the rounds fired". Would you like to provide the methodology the Luftwaffe used to determine this percentage? Was it the poor quality gun camera images, remaining rounds in returning fighters versus hole counts in downed bombers. I will to bet a beer it was not analysis of only firing passes from head on. "...
I don't know the methodology, The figures are from one of Tony Williams's books. It may have been counting the holes in downed bombers. There was a number for head on passes, while I didn't post the numbers I did say it was a lot fewer in a previous post. The problem(s) with relying on the head on pass are 1. it takes more skill, closing speeds are higher than any other form of attack and give less aiming time and firing time. 2. The Germans developed this attack against B-17Es Fs and early B-24s. which had rather weak nose armament. The head on attack may have diminished in frequency as/after the powered nose turrets were introduced. Maybe somebody with more knowledge of the operational histories can help here?* 3. If the head on attack truly becomes the preferred method increasing the protection at the front of the cockpit is an option.
My position is: Under the conditions of using a single engine fighter to attack U.S. heavy bomber boxes, head on passes with heavy machine guns may have been more effective due to more numerous sufficiently powerful projectiles than the primarily cannon armament of fewer more powerful projectiles that was actually used.
It may also have been a better tactic to use single engine fighters with HMG and larger magazines to attack the escorts so the twin engined fighters with cannons would not be harassed when firing shells out of the effective range of the bombers defending .50s
The Germans certainly had the most experience in trying to shoot down 4 engine bombers. They also were in a position to capture&test a wide variety of aircraft weapons. Not only their own but US, British, Russian and Italian. Which leaves out the Japanese, who are the only other major player. Germans also had access to Swiss and Hungarian guns. Now maybe they did draw the wrong conclusions but several things have to be gotten out of the way for that to be true.
1. what is the weight of the installed armament and ammunition?
Just the weight of 2000rounds of .50 cal ammo is 622lbs. 4 guns worth at 500rpg. While a 190 could carry it it is a bit beyond the ability of a 109. You still need the guns, mounts, heaters, ammo boxes , etc.
going to a less extreme load we find the P-51 could carry a max of 564lbs of ammo and the guns, ammo and armament provisions total around 1200lbs.
More firing time than a 190 for sure but are six .50s really better than 4 20mm MG 151s plus 2 mgs? If the 190 has the MG 131s in cowl the total rounds per second is very close so strikes per second are going to be close. Destructive power is in favor of the German guns (even the 13 mm projectiles have HE or incendiary).
2. As a continuation of the above. what is the effectiveness of the armament vs the weight of the installed armament. The american .50 cal guns were heavy for their caliber and the american .50 cal ammo, while high velocity suffered, as all high velocity rounds do, by being heavy for the size/weight of the projectile.
3. Not only the Germans but the Japanese were moving to bigger guns for destroying bombers. British were standardizing on 4 20mm cannon. Soviets were moving to bigger/more cannon. Was ONLY the USAAF correct in sticking with the machineguns?
And the USAAF was specifying 20mm guns for night fighters (some P-70s and the P-61s) and bomber destroying aircraft. While we may have used six .50s in our fighters in Korea the F-87, F-88, F-89 and F-90 were all specified with 4-6 20mm cannon several years before the fighting broke out in Korea.