Comments by "" (@neutronalchemist3241) on "Thompson 1921: The Original Chicago Typewriter" video.

That was not the last time. It had been only during WWII that someone bothered to see if the .30 Carbine round could be conveniently fired in a blowback sytem, and discovered that a 570 grams bolt was enough even for proofloads. All the M1 carbine manufactured, with their gas systems and rotating bolts, could have been replaced by much cheaper to produce, simpler to service, and even more reliable, blowback weapons.
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It's actually quite surprising none noticed, during the development of the weapon, that the Blish lock was completely useless, and so, to leave it, would have added unnecessary complexity to the gun. However, that was not the last time. It had been only during WWII that someone bothered to see if the .30 Carbine round could be conveniently fired in a blowback sytem, and discovered that a 570 grams bolt was enough even for proofloads. All the M1 carbine manufactured, with their gas systems and rotating bolts, could have been replaced by much cheaper to produce, simpler to service, and even more reliable, blowback weapons.
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The Thompson more or les worked, but the price to produce a SMG that simply worked was way too high, and it was very heavy for what it did (the M3 was an entire kg lighter).
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