One of the common misperceptions is that the gas tube somehow dumps an extraordinary amount of gas residue into the action. First, it does direct gas into the bolt carrier - which IS the gas cylinder and piston. Every one I have cleaned from the Garand up will get residue into the gas cylinder, it's like they designed it that way or something.
Same for the AR15.
What also happens is that, correctly timed, the bolt unlocks and starts retracting as it first part of the reloading cycle. There will be residual gas pressure in the barrel when this happens - it's trying to exhaust up to 11,000 psi at that point, and it all can't get out past the muzzle, any more than your tire with a nail in it can. It leaks down a little more slowly that a sudden explosive decompression, thank goodness. And since the bolt is unlocked and the brass is pushing on the bolt face, once it's moved just a fraction of an inch, gas pressure rushes past the casing. As it keeps retracting, the hole gets bigger until it's base diameter sized, dumping a lot of gas residue onto the bolt, carrier, and trigger parts. Not the tiny little tube high up in the receiver connected to a .061" or slightly larger gas port.
So, the tube feeds the gas cylinder inside the bolt carrier, and the chamber dumps most of the residue into the action. And that's the way it is with ALL self loading guns. Even blowback .22's - you eject dirty brass and have to clean them, too. No gas tubes or nuthin. If you have owned a roller locked HK you certainly experienced it - again, no gas action at all, dirty brass and dirty action. I had to clean more residue out of it when I owned one firing a mag of .308 than a couple of mags in my 5.56's. Government powders are the cheapest bid and aren't known for cleanliness. They do that because a battle weapon requires daily maintenance No Matter What just to clean off the mud, rain, dust, and lunch. Dragging it thru the local environment with three second rushes, low crawling, crossing streams and ditches - you clean it daily or else. Gas residue isn't the real problem in combat, and a basic load of 10-15 30 round mags won't dirty it up enough to cause a stoppage.
In the field it gets a bit of oil, you wipe it down with the tail of your t-shirt, and you do it during your assigned down time on overwatch. That is the standard for a combat gun, and the M16 design allows you to get to the gas cylinder in seconds, unlike the armorer level machinations involved on typical piston designs. And as for a piston AR - be aware, every one is proprietary. You can only buy spare parts from the maker, and there are already a few who have gone out of business. It's a boutique adaptation, not combat proven and certainly out of the mainstream when you need to fix it.
We went to this design decades ago, when we had 1 in 10 who were service veterans and every one trained in maintaining a piston gun. It wasn't wrong to say the M16 was a gun that "didn't need cleaning" when compared to the much longer down time of the Garand or M14. It was a fraction of the time needed and didn't involve special tools and a ground mat to do it. What got a fly in the ointment was ramping up production four fold, bad chambers in outsourced barrels, and powder lots which were repurposed from their intended use in much larger weapons. McNamara and the whiz kids ARE to blame, it's what happens when you let people who are not qualified subject matter experts to make decisions.
Like that has changed for the better? Well, in the Army, yes, even newly minted Generals get instruction from Specialists on their area of expertise. This is why it's important to largely discard those talking heads who once wore stars when it comes to subject matter expertise - and when it comes down to gun design, to listing to gun designers and engineers, not users who have no clue what is going on after they pull the trigger.
ALL SELF LOADING GUNS DIRTY THE ACTION. Its a necessary offset of their design and there's no getting around it. If you want a clean action and clean brass - keep the bolt closed until all the gas goes out the muzzle, and operate it manually. There is no other choice.
