In October of 1940, the US proving ground at Aberdeen, Maryland conducted testing of both the Madsen light machine gun (in 7mm caliber) and the Madsen anti-tank/anti-aircraft cannon in 20mm. On November 8th it published a report on the trial (according to its title, the 143rd such report on machine gun testing – I would love to find a full archive of those reports!) which included function testing the LMG, armor penetration testing of the 20mm, and testing the effectiveness of the explosive warhead in the 20mm projectiles. The report is 17 pages of text and a further 35 pages of photos, and should be interesting to anyone interested in the topic.
In particular, the armor penetration testing includes notes on the exact thickness, hardness, slope, etc of the armor plate used. It was noted in the conclusions that the 20mm Madsen was not as effective as the 20mm Solothrun previously tested (which, unlike the Madsen, could pierce 1.5″ face-hardened plate at 100 yards).
You can download the complete report here: