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As the US entered WW2, there was a lot of things they lacked in equipment, this included submachineguns, and the only available domestic option at the time was the Thompson, which works, was in .45 Auto, and existed in quantity, but is slightly heavy, and more importantly was rather *expensive*.
As soon as it was adopted, it was realized that the Thompson was taking too much time to make, and was costing way too much money, and good effort was put into making it more economical.
Among the things to get rid of was the 'Blish-Lock' setup, which was based on a false premise on the physics of metals and friction, which in actuality just worked as a shitty delayed-blowback mechanism, with an angled brass block riding in the steel bolt of the gun. This mechanic would really just barely delay opening of the action, and break rather often, and was eventually deleted from the guns, turning the bolt into just a solid lump of steel instead, which worked MUCH better.
Other things included the sights, John Thompson had originally wanted expensive and fully adjustable target sights, which are nice in recreational sense, but pointless for a military subgun, these were axed from the spec and it got a simple shielded peep sight, the gorgeous rustbluing was another needless expenditure and was binned as well, along with the cooling fins on the barrel, the vertical foregrip, the drums and their cutout, etc, etc.
The eventual M1A1 spec had taken the 1928 Thompson, which in every sense was a first generation subgun, and made it as economical and practical as possible, cutting production cost in half, made production time faster, and manufacture was also contracted out to additional firms such as Savage Arms and others.
Still, they needed more subguns than this, and faster, and the gun is still pretty expensive.