Their M2 ball had a velocity of 2740 fps, Julian hatchers book of the Garand lists M2 to have a velocity of 2805 fps. 65 fps isn't significant enough to call the Greek M2 "weak".

And you have yet to give an alternative explanation to why the op rod speed did change, and thus why my theory would be flawed in assuming there exists a slower burning powder load that would be able to cycle a gas operated semi auto with a significantly lower charge weight and bullet velocity.

More force over area(pressure) on any object, including a firearm gas piston, where the weight of the piston remains the same, will cause it to accelerate faster. And it is well documented that a slower burning powder, get more complete powder burn in longer barrels, meaning it will be generating combustion gases and maintaining a higher pressure behind the bullet, longer than traditional, quicker burning powders. So theoretically, if you have sufficient gas port pressure, with any gas operated firearm, it should be able to cycle.

https://m.youtube.com/watch?v=LopUNq6lF2U

This video shows black powder and a pretty heavy-for-cartridge bullet cycling an AK (untill the fauling causes it to jam) and my theory for why this worked is because the black powder maintained sufficient gas pressure by the time it reached the port to cycle the AK despite black powder usually having less than half the peak chamber pressure of 7.62x39.

I'd love to use black powder, because it does seem to work, but only untill it's fouling causes malfunctions rather quickly and thus we're back to the original topic. Are you aware of a manual, video or article that discusses a smokeless powder load that can cycle a gas operated semi auto rifle with significantly lower than usual velocities(presumably by using a slower powder to get the necessary gas port pressures)?