The comments have degraded into DI vs Piston gun design trolling.
I even commented...
Do you realize that the AR isn't actually direct impingement?
Have a look that the bolt carrier on an AG42 or MAS49 and you will see that the gas strikes or impinges on (hence the term) a pocket on the carrier's face and that sends it to the rear.
The AR has an expansion chamber inside the bolt carrier, and the bolt is a piston. The gas is not striking the bolt carrier, but passing through the gas key and into the chamber inside the bolt.
Time to stop using the term incorrectly in the gun world.
As far as popularity goes? I think that the only guns that use the Stoner gas system are AR's either AR15 or AR10 and their derivatives. And that's presently most of the 5.56 guns in existence for 50-25% the cost. Which is the real reason they've the taint of popularity against piston guns.
$800 AR or $2,400 SCAR? Quantify three times better than an AR and do it in terms of ammunition I might likely carry before any substantial difference in reliability will surface. Document it, show your work, PROVE the claim.
Or a claim is all it is. An anecdote.
Further down the comment chain someone is claiming that their piston gun saves them an hour of cleaning time.
What are you using for cleaning solvents?
This is something that's come up in my anecdotal experience too. With a piston gun, no less.
There's an FN MAG machine gun coaxial to the main gun on an M1 Abrams. It's called an M240 in the US system. Using the issue Break-Free® CLP it takes hours to get that thing clean after a day on the tank tables. This is because CLP's tend to be OK at each of the Cleaning, Lubricating and Protecting; but not actually good or great at any and normally much better at one than the other two. Break-Free® CLP has an emphasis on the L. It's an abysmal C. LSA is a better C and it was developed when the Army issued three different products for doing CLP (Rifle Bore Cleaner, LSA and Cosmoline).
I don't have a MAG to clean with modern cleaning products. But I do have ARs. Break-Free® CLP took considerable time and effort to get an M16A2 clean; and we were tankers, we didn't shoot the M16 often or much. Hoppes No 9? Nearly no effort; but you have to lubricate the gun with a different product once it's clean. And there isn't an hour's worth of cleaning to do.
Thus far the only CLP I've tried that seems to be pretty darned good at all three things is Frog Lube. I didn't want it to be so either. It ruins my, "C, L or P; pick one,"pithy line.
Still, if you want the carbon gone, a carbon solvent designed purely as such, will work faster and easier than any CLP out there. Hoppe No 9 and Shooter's Choice are my favorites. They're even copper solvents too.
Shooter's Choice and LSA cleaned a 1957 Ruger Standard that apparently had not been cleaned since it left the factory in about an hour. You just don't get dirtier than a blow-back .22. The main difficulty is getting to all the nooks and crannies to lift the dissolved carbon and not getting it to dissolve when you're using good solvents.
This comment thread raised the reliability thing again.
And again I am struck with, "how much ammo do you think you're going to be carrying?"
210 is the basic US infantry load. That's a VERY low bar to pass. How many rounds without cleaning is reliable enough? Twice the basic load? Five times the basic load? Ten times the basic load? 210 rounds is 7 pounds of magazines and ammunition. How much are you bringing?
And how subjective will our reliability tests be? The obsolete and unreliable (if you read modern accounts) M1911 could barely choke down 6,000 rounds of ball ammo in acceptance testing and a randomly selected gun from each lot had to do 5,000 or the lot was rejected. Issue load for anyone who got handed a 1911 in the US Army was a whopping three magazines. 22 rounds tops.
The main reliability problem with the 1911 in actual use was it rusting solid from being kept in a leather holster very rarely drawn let alone fired.
The more I learn about guns first hand the less I believe the internet. The more I talk to people who were actually in combat with a particular weapon, the less I believe the internet.