At which point is one weapon merely a variant of another? Isn't this change in the action a significant improvement?
[1] https://en.wikipedia.org/wiki/M27_Infantry_Automatic_Rifle
In fact, many such piston AR retrofits exist in the US civilian AR market, but they’re generally thought to be of “unproven” longevity, at least.
The advantages of the retrofit piston being slim and light is that the recoil increase is small. The disadvantage is that durability is questionable, as those pistons experience a lot of violent force and most guns designed from scratch to be piston operated have significantly heavier duty pistons, for good reason.
You can read online of Marine armorers lamenting how their M27s are breaking down at a rate much faster than their M4s. This is of course anecdotal, but it is also not entirely worthless data.
The "direct impingement" system of the M16 / AR15 isn't really true direct impingement, as the bolt itself works as a gas piston within the bolt carrier. But the gas has to travel much further to reach it than with a conventional piston.