Where you put the piston is entire up to the buyer. What goes along with making that decision is whether the marketing and fanboys claims actually stand up.
Cleaner: It's claimed the piston runs a cleaner action, but in reality, all self loading actions pass gas around the case as they unlock, dumping it on the bolt. This simple fact is disengenously ignored by piston fans. I'd like to find the pic of the AK bolt seriously encrusted with gas residue after 2,000 rounds - as long as the action is opening before all the gas pressure is exhausted from the barrel, then gas will contaminate the action. What's important here is that a soldier will lube it and wipe it down daily, mandatory because of the environment.
Piston compared to piston, they both get equally dirty - and it's been proved more than once, you can neglect them equally, too. If that's your choice, fine, but claiming the piston runs cleaner isn't necessarily so. And trying to clean it is more difficult - you can't shotgun the action and pull the bolt out of the carrier in 30 seconds. You will fight the handguard or likely, the attached free float, off the gun to get to it.
Cooler: yes, a piston gun runs cooler at the bolt face and carrier. DI warms up about 50-80 degrees more. That's it, you can still shotgun it and disassemble the bolt from the carrier barehanded. BUT, heat at the piston on the barrel has been measured above 400 degrees, requiring the use of a high temp handguard, usually an aluminum free float. Piston gun users are commonly reporting using a glove on the off hand because of it.
Reliability: No documented testing. It's expensive. Just marketing claims that are largely biased attempts to magnify the old rumors of gas residue jamming the DI action. That's significant because it IS rumor - the major cause of stoppages reported by the military and most professional users is magazines, ammo, and user incompetence - not gas residue. That will be exactly the same for a piston gun, too - there's no effective difference, they use the identical magazine, ammo, and shooter.
Tilting carriers: the M16 was designed to use gas pressure to push the carrier straight back on its axis. It even uses that same pressure to counteract the case head pressing against the bolt face to help the lugs turn and unlock. When a piston design is adapted, it usually is attached at the gas key, which was never designed for the thrust. It's offset from the axis of the carrier, which does tilt it. Many piston gun makers now use specially designed carriers to compensate, with additional material added to the carrier to limit this tilt. In a purpose built gas piston gun, the bolt carrier rides on rails to control it - and it's not a problem. AR = no rails = modified carriers to reduce the tilt.
The three major "advantages" of a piston AR are mentioned over and over to help the makers SELL GUNS. It's about protecting your marketing position, and being different in a saturated market means putting out something more than another Me, Too! gun. Pistons are a modification of a gun that was designed to eliminate the op rod and it's disadvantages - losing the parts makes the gun simpler, more reliable, and has more design efficiency.
It's usually brought up and explained carefully in most DI vs Piston threads that the three claims of running cooler, cleaner, and more reliably aren't really substantiated. The actual point is to defend the selling of $400 more in parts and giving it the "Rolex" spin to justify what you get - a status symbol at the range. If someone has already drank the piston koolaid then changing their mind is long past accomplishing. Enjoy and have fun, just don't expect us to join in with the same relish. We'll be in the next lane outshooting you, skill counts for more when there's really no difference in the tools.
And that's what she told me.
