Direct impingement and pistons both use pressure from the fired cartridges expanding gases to affect the bolt carrier group.
The M4 Shits where it eats and the 416 shit in the door. Either way your going to be cleaning and scraping carbon residue.
Since I have never cleaned a 416 I can't tell you how much trouble it is to clean the carbon form up front and how quickly it may block the gas ports.
The piston helps in one regard. The bolt carrier group is not subjected to hot gases being dumped through it so it is cooler and lowers the chance of cook offs. If one can't help themselves trying to use and M4 or M4A1 as a light MG.
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