Along with Wright and the R-3350 engine, Torpedoes, Tank and Tank Destroyer policy, the failure of Ordnance Department to pull their collective heads out of their asses are among the top failures
of WWII
And the 20mm wasn't even the worst Ordnance did
The short list
1.
a 'light' MG in the form of the 32 pound M1919A6
Weight was less ammo, too. This turd lasted till 1957, despite the Army wanting a LMG since 1922, that was a bit better than the BAR
Despite having access to LMGs and the German GPMGs, this was the best that was done, leading to #2
2.
inability to convert the MG-42 to run on .30-06. Headspace issues, indeed.
3.
Wasted the entire war on the HMG .60 cal Machine Gun and the early part on an Anti-Tank Rifle that never saw service.
It started life as a necked down Navy 1.1" cartridge.
From the US History on this round
In 1939 the Army developed a caliber .60 antitank cartridge. Early in World War II our ordnance engineers anticipated a need for a machine gun heavier than our caliber .50 Browning and began work on this caliber .60 which would fire a 1200-grain projectile at the then “hypervelocity” of 3500 fps.
This round was later necked down to caliber .50 and achieved a velocity of 3900 fps! Later yet, it was necked up to 20 mm, known as the 60/20, and fired a 1500-grain projectile at 3300 fps. This round gradually evolved into the M50-series which is now the most widely used 20 mm ammunition in the world.
So yeah. only took 15 years, and using a gatling setup to get things working.
4. 20mm Hispano. See #2. another case of taking a weapon that worked, and made many that rarely worked.
5.
The BAR. Pretty good for an automatic rifle from 1918. A number of countries, the Swedes, Poles and Belgians, improved it to being lighter, QD Barrels, and even belt feed.
Ordnance worked on the bipod and carry handle. Woohoo!