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The M1 which entered service was using a so called "gas-trap" setup, known primarily from Danish inventor Søren Bang, where instead of drilling a port in the barrel to tap gas from, a concept which a lot of people were paranoid about in that day and age, there's instead a sliding inverted cup at the front of the muzzle. The bullet exists the muzzle and fills the cup with gas, which pulls it forward to actuate an action bar or similar.
This sort of works, but not well, it's fidgety and the gasses leaving the bore into the larger and less sealed cup cools them down and produces more carbon which has no choice but layer in there, so a gas-trap action will actually get very dirty, very fast, you could not be expected to get past 100rds in ideal conditions before you start seeing problems. Some time into service use, this eventually gets faced, and the decision is made to upgrade/retrofit the M1.
By 1939 it gets a more sane setup with a port drilled into the barrel which acts like a more conventional (in hindsight) long-stroke piston, and which to the surprise of doubters, doesn't reduce the lifespan of a rifle barrel, doesn't worsen accuracy, doesn't dirty easier (it dirties far less and is way easier to clean), and is 100% an improvement. The US Army gets into retrofitting the existing rifles as well as updating the production, and though smaller numbers of earlier gas-trap rifles did make it into the European theater in WW2 in rear echelon places, the vast majority of M1 rifles which were originally built with the Bang system got retrofitted, so unmodified examples are actually incredibly rare today.
That a rifle employing a jank-ass setup like this would end up readily trouncing the other contestants kind of paints a picture of how well THOSE worked. Thompson's rifle required oiled felt pads, while Pedersen's used an outside wax coating on the ammunition which liquified from the heat of discharging, which is comparatively more refined, but neither are good.
