isnt a car a4 a flattop carbine?
What are you asking? There's no "piston" in a real AR15 or the Carbeanated version!
Wallery, welcome aboard,
Up top there you'll find an "INFORMATION" button, good stuff there for you to read.
At the top of the AR DISCUSSION, BUILD IT YOUSELF & AMMUNITION forums your will also find some excellent threads tacked, there especially for you s so you can read and get up to speed.
ETA - this post will probably get moved or trashed as you posted it in the wrong forum, so paying attention to the subject of question and posting it in the proper forum may help avoid these issues too...
A "carbine" is simply a slightly-shorter-barreled rifle.
Also, when the high-pressure gas reaches the bolt carrier, it quickly looses its pressure. The bolt carrier gets a "punch" of gas, but that "punch" is strong enough to operate the ejection and reload functions.
Here is how it works in all AR's that are direct gas (not piston).
1: Rifle aimed in safe direction and a loaded magazine is inserted.
2: Pull back on charging handle and release.
3: Bolt and Bolt Carrier move forward and strip a round out of the mag and load it in to the chamber.
4: Once the round is in the chamber the bolt stops forward moment and starts a counter-clockwise rotation locking the bolt in the barrel extension. This is caused by the cam pin and the continued forward motion of the bolt carrier.
5: Safety moved from Safe to Fire.
6: Rifle aimed in safe direction.
7: Trigger squeezed.
8: Hammer is released and strikes the fire pin igniting the round.
9: Bullet travels through the barrel passing the gas port in the barrel.
10: Expanding gases travel up the front sight base and in to the gas tube traveling back to the gas key on the bolt carrier.
11: The gas enters the bolt carrier pressurizing the chamber created by the rear of the bolt in the bolt carrier. This pressure is pushing the bolt forward (just holding it forward) and the bolt carrier reward.
12: The bolt carrier starts a reward motion causing the cam pin to rotate the bolt clockwise, starting the unlocking cycle and the extractor is pulling the fire round out of the chamber.
13: Once the bolt moves back far enough to cause the bolt to unlock the lugs the gas rings on the bolt are now passing the gas vents on the right side of the bolt carrier releasing the pressure from the bolt carrier chamber.
14: The bolt carrier continues its reward motion pushing the hammer back in to its cocked position.
15: Once the bolt carrier has moved back far enough the round is ejected out the ejection port by the ejector in the bolt face.
16: The bolt carrier continues its reward motion until it is stopped by the buffer stopping in the buffer tube.
17: The bolt carrier is then pushed forward by the buffer spring loading the next round.
18: The disconnctor is now holding the hammer down until the trigger is release and the hammer moves forward being caught by the trigger sear.
19: The AR is now ready to be fired again.
(I hope I didnt miss anything)