The M-6 was also, unlike most U.S. designs that actually reached low scale production, an utter POS. 57 TONS to support a 76mm gun (oh, AND a 37mm in the same turret). Thank God they only built 40 production versions.
It was an American Tiger.
Unlike the Tiger, it was mostly reliable, but not as reliable as the M3 or M4. Few 50+ ton vehicles were. But it predated the Tiger, it was a contemporary of the M4 Medium
The T1E2 pilot build by Baldwin tank was running in September 1941, with testing in August, just over a year after the Heavy tank project was greenlit in October 1940
Now in 1940, the 90mm gun was not ready yet, so the Army used the most powerful gun they had, the 3" T9 AAA.
The end of May 1941, Adolf ordered Porsche and Henschel to have 45 ton Tigers prototypes running by his birthday in 1942, and so, two very unreliable prototypes were shown off to AH
Unlike the Tiger again, the US had not had the experience of bouncing 50mm rounds off of KV-1 tanks, so didn't see the need for heavy tanks like the Nazis did, and trials went slowly from low priority, with the Standardization only occurring in April 1942. Thousands of Orders slashed to 230, then again to 40. First Production M6 were done in December, 1942. Ordnance Department had been pushing to put the new T7 90mm gun into the M6, and fitted one to the T1E1 pilot, but the Heavy Tank program was heading for cancellation.
Normally, folks want to blame McNair for this, but this one was on Devers, CO of Armored Force.
The US should have had some Heavy tanks, and in combat in 1942, when improved versions could planned for. As it was, 90 and even HV 105 mm gut were fitted.
It was an outstanding idea for 1940.
Compare to the Churchill. Would rather be in a Churchill I or even III, when you could be in an M6 in 1942?