Smyth Busters: The AR-15, Direct Impingement or Piston Operated?

Today, Brownells Gun Techs™ Steve Ostrem and ‪@CalebSavant‬ want to settle a debate. Or maybe just stir the pot. The debate among AR-15 fans is that the AR-15 platform actually DOES NOT have a direct impingement operating system. Steve asks the pertinent question: So where's the piston then? Caleb gets technical and starts by defining terms.
A traditional GAS PISTON operating system - like on the AK series, FN FAL, HK416, or BRN-4 - sends combustion gas through a gas port in the barrel into a sleeve that applies pressure on a piston that pushes on the carrier to unlock the bolt. In a DIRECT IMPINGEMENT operating system, instead of the gas pushing on a piston, it goes down a gas tube and pushes directly on the carrier to cycle it.
The AR-15 works a little differently. The gas goes THROUGH the carrier and pushes directly on the BOLT to move it. The carrier has two holes that vent the gas out of the carrier and out the ejection port. This helps minimize muzzle rise, which can be quite pronounced on a pure direct impingement system. But the big news is that the bolt acts like a piston and the carrier acts like the sleeve. So the AR-15's operating system really IS a lot like a traditional piston system!
Pure direct impingement rifles like the old Swedish AG42 or French MAS-49 get a lot of carbon fouling on the carrier because the gas doesn't get vented out like it does on the AR-15. The AR-15's bolt and carrier still get dirtier than those of a traditional piston-operated rifle, which keeps all the gas (and all the fouling) up front in the gas block.
So the "myth" is not busted. The AR-15 actually is not a true direct impingement rifle because the bolt itself acts as the piston and the carrier acts as the sleeve.

[@CalebSavant]

Merry Christmas everyone! I gotta give a shoutout to Rad Labs 3d for the sweet custom Brownells AR stand. Oh, and if you haven't yet, go watch Violent Night the movie.

[@themandan4000]

When are we getting the sexy christmas calender

[@US2A]

I want a cut in half AR

[@user-mj1vb8jr6j]

I’ll second the calendar request. I wanna see kaleb all cute as Mr February

[@MegaAndy93]

Internal piston doesn't sound catchy. We have short stroke and long stroke, why not call is the Stoner Stroke Piston (LP, SP, SSP for dem acronyms)?

[@CalebSavant]

@@PAIsIllegal probably not one worth owning honestly. A barrel is not something you want to cheap out on. There are affordable barrels that are good but I would stay away from hunting "dirt cheap" when it comes to key components in general.

[@GNpatent]

Stoner was a freaking genius. The AR is basically a tube gun, no reciprocal mass that is of the bore axis except for the tiny hollow gas key.

[@freeandcriticalthinker4431]

What do you mean? The bolt is reciprocating.

[@Lucas12v]

@@freeandcriticalthinker4431 I think he meant to say that all the reciprocal mass is inline with the bore. Other than the gas key.

[@thegreatshitstain]

*OFF the bore axis

[@recoiler4545]

@@freeandcriticalthinker4431 I think he meant to type, "no reciprocal mass that is ABOVE the bore axis except for the tiny hollow gas key." Being "above the bore axis" is the KEY thing. Hah, pun! 67

[@mikeoxlong3676]

Kalashnikov was more genius.

[@middleclassthrash]

I like how Steve tries to play the dummy role, while we all know he's very intelligent.

[@akcarlos]

if anything he has probably known this all of his working life.

[@silent1967]

I like how he just gives that ho-hum look and takes another drink of coffee ☕️.

[@jetboy770371]

I think the gray haired guy hits that coffee cup a little too much. This video should be used for a drinking game.

[@jetboy770371]

@silent1967 After they stop the video shoot, that coffee swilling guy has to take a serious piss

[@silent1967]

@@jetboy770371 Don't worry, when you get as old as he is you'll be pissing a lot too, whether you drink coffee or not.

[@dv8322]

Again, Steve and Caleb are the best father son duo in the world. Steve, you raised one hell of a kid and Caleb you are lucky to have such a great dad. Love you guys. 😁

[@CalebSavant]

Stop it... lol Steve is not my dad.

[@dv8322]

@@CalebSavant So cute.

[@jwilsonhandmadeknives2760]

@@CalebSavant keeping up the kayfabe for the viewers. respect. but isn’t it weird calling your dad by his first name?

[@ShepherdOfTheSilkies]

@@CalebSavant I’ve been seeing these comments for a while and I’ve been waiting for you to respond😂 don’t forget the last people the gun community did this to head to go get a paternity test

[@user-mj1vb8jr6j]

Hahaha Steve is a great dad. He’s so patient with Kaleb, I’d loose my mind if I were in his shoes.

[@Axe_six-six]

A couple years ago, I heard Chris from Small Arms Solutions refer to it as an "internal piston" system. At first, it went against everything I had been taught about the system since I was issued my first M16A1 in the early 80's. Then he explained it much the way Caleb did, then the light bulb clicked on. I've referred to it as internal piston operated ever since. If I'm not mistaken, I believe there is old interview with Mr. Stoner where he calls it internal piston also. But I'm not 100% sure on that. Great video guys, as always!

[@Ni999]

Pistons are contained by cylinders. They are always internal. You had it right the first time. The entire bolt-is-a-piston story came from the patent attorneys and their creative writing - not Stoner. Ever heard of the cylinder being driven and the piston being dragged along? No. No you have not, except for the AR fiction

[@woodrowcall3158]

@@Ni999 Have you ever driven a SuperMoto?

[@Ni999]

@@woodrowcall3158 No and not all supermotos use rotary engines. See my other reply. The AR bolt is still not a piston.

[@BFTR]

So, it's an internal piston system???

[@hairydogstail]

@@Ni999 So it is a cup piston, much like the LWRCI piston only the carrier is the cup piston..

[@hamzaimran7086]

You guys explain everything so well that even a novice can understand completely.

[@dtmelanson]

That is the real genius of these videos.

[@leonardwei3914]

"AR-15, Direct Impingement or Piston Operated?" AR-15: "Yes"

[@markmartinez7895]

Great vid. Would love a part two about cost differences, reliability of gas piston and more parts to break, weight and recoil differences as well.

[@iamgermane]

See the Adams Arms video. DI vs there piston upper. The DI upper is dirty as hell and runs hotter!

[@ForOrAgainstUs]

"Oh hey Steve, didn't see ya there." It's the little goofy things I love about these videos.

[@speedrrracer]

That's some great AR education, many thanks

[@Patrick-is4ym]

It’s a piston gun disguised as a direct impingement gun, lol

[@Lorenzo-fw3gx]

When Caleb's pomade was made, Eugene Stoner was still sketching the AR design...🙃

[@wfeiereisen]

Great videos. Not only informative, but entertaining.

[@rhinox0110]

I've heard this described several times before and this is a good description with visual aids. Thanks guys much appreciated.

[@emmanuelgoldstein3682]

According to Eugene Stoner: the bolt is the piston.

[@PHIllip324]

I sometimes refer to rifles like the AR-15 and the Daewoo K1 as being "internal piston" rifles, as opposed to "external piston" systems like the AK-47 or the HK-416.

[@vensheaalara]

"I'm not here to end it" and "that's a lot of syllables" totally got me. Lol. Good stuff.

[@chad9166]

Definitely a very elegant solution, seems like there's always something new to learn about the AR platform

[@criagcundiff2469]

Thank you very much for this and all the other explanations. The info you guys provide really helps me understand the workings of my firearms. And the differences between many that seem very similar.

[@mattkemerait]

I love these guys. The dynamic duo of Guntube.

[@jamescarter4175]

``Your bolt acts as the piston...''. That is THE difference between the AR-15 platform and a piston driven platform like an AK.

[@denniscasey986]

Great teaching. Thank you.

[@7sq]

Great information ,good comedy ( thx 4 all of the videos ) !

[@MR619Can]

"This invention is a true expanding gas system instead of the conventional impinging gas system." - direct gas, expansion, bleed off system

[@hairydogstail]

It is a piston, as described in Stoner's patent..

[@MR619Can]

​@@hairydogstail I didn't say it wasn't. Just pointed out that it isn't an impingement system.

[@ChampionofVardenfell]

THANK YOU!! I have been hearing people argue this point back and forth for YEARS, but none of them have been able to actually explain what direct impingement OR piston driven even MEANS. And since I didn't know myself, all I could do was smile and nod. Glad someone finally came along to explain it and put this shit to bed.

[@jackshett]

And this is where you get adjustable gas bolt carriers as well. Though, there aren't many of them.

[@dtmelanson]

I want to be able to like the Steve and Caleb videos more than once.

[@rainmaker1346]

very good info guys. Thanks a lot.

[@augreich]

I would agree that the AR is technically a piston system.

[@monsterk7603]

Love these videos.

[@michaellogie]

Merry Christmas to my favorite firearms duo🎄🌟

[@andycraddock7677]

Just when I was finally understanding the difference between a direct blowback v. gas-recoil operated pistol (And at an elementary level at that) Steve and Caleb drop this one on all viewers! Good, interesting video (as always from the Smyth Busters duo), but I’m sure I’m not the only viewer still not certain I could explain the differences between the two operating systems, even after viewing.

[@tedhodge4830]

It literally just comes down to where the gas is being introduced into the mechanism. At the chamber, at the gas key, at the piston face, at the bolt face, etc. The gas tube on an AR-15 is just a tube that dumps the gas into the receiver, that's it.

[@andycraddock7677]

@@tedhodge4830 : TY Mr. Hodge. Happy Holidays.

[@JeremiahHartmanPhotography]

Great Video! I have both (DI"ish" and Piston).. I love both for different reasons. Piston definitely handles suppressors much better, and can operate even if fully submerged in water. My DI rifles are great as well, lighter recoil, flip is manageable with a muzzle brake. Both are great designs.

[@baum6721]

Great video! Also, love the Ryan George mug Steve's got there.

[@RobinP556]

Great video. I think that we can lose our minds delving into the absolute best term for everything. Back when I was in Army as an 18B we were taught that there were 2 types of gas piston systems (gas piston & gas tappet) now it’s normally referred to as long stroke gas piston and short stroke gas piston, which in my opinion makes less sense but it is what it is. As long as someone that should be in the know doesn’t call a “magazine” a “clip” or a “cartridge” a “bullet” I’m good, and there are plenty of high profile gun YT channels that use the term bullet when referring to a cartridge.

[@AfternoonProductions]

How did you guys know I was arguing with my brother about this on christmas?! I'm late but wanted to drop by and say merry christmas and happy new year to Steve, Caleb and the Brownells crew! PS. My dad already had Die Hard on when I arrived for our Christmas dinner.. from NH, Live free or Die!

[@ellerybice3787]

Live free or die needs updating, I want to live free and live. I propose the following change. Live free or die fighting. & Give me liberty or give me death needs updating as well. I have Liberty, death can wait.

[@AfternoonProductions]

@@ellerybice3787 The sentiment is there, it reads a little harsh out of context but I like the truth behind it. I wish NH would bring in the Gadsden license plates like Virginia, the old man on the mountain is, well, old. Lol

[@Alex-12381]

Not only is it a piston gun, it's a long stroke piston since the piston travels the entire distance with the carrier

[@dieudonnesaive6603]

Why would you say that?! 😂

[@hairydogstail]

@@dieudonnesaive6603 Re-read what he wrote..

[@Alex-12381]

@@dieudonnesaive6603 Because it kind of is technically true, it's just missing the operating rod lol

[@dieudonnesaive6603]

@@Alex-12381 technically since the bolt isn’t fixed to the bolt carrier, I’m not sure it’s exactly true? The piston (bolt) is its own moving part aside from the carrier. It really just depends on what definition of “long stroke” we roll with. There’s a hundred different ones out there, all of them kinda flawed in one way or another.

[@wingman8447]

Amazing. Genius stuff

[@jknewb6276]

Interesting knowledge right here, thank you gentlemen!!

[@SoloRenegade]

DI has some advantages like self-cleaning of debris too.

[@brodyscarlett5527]

Fueling an online gun argument? Classic Caleb moment. Great video BTW

[@horatiodreamt]

Excellent vid.

[@jameskennerly9748]

Awesome Sirs!!

[@brent9278]

Who knew that Steve is a fan of Pitch Meetings? Super Easy, barely an inconvenience!

[@chriserickson4417]

Who am I to disagree? I work at a Gun Range and am always learning from our Gunsmith, so I am not going to argue any points concerning operating systems, etc., but will be watching and learning what I can from you guys (Steve and Caleb). Thanks again for another great video.

[@scottmcfee1665]

Hi Love your videos. Would you think about discussing the Noreen gas system they use on their AR style 30-06 and 338 models??

[@jimmyjimenez6591]

It's pretty cool to see and hear it clearly explained 👍 With that better understood by me, I now get to wondering if Ruger's Mini-14 is actually best described as a direct impingement system gun? 🤔

[@UnknownUser-fe5zu]

As a motorcycle technician, I think I know a piston when I see one. 😂

[@03redrubi]

Great deep dive into the DI gas system. One could argue, a lot of the heat is dissipated over the entirety of the DI gas system, vs all heat collected at the gas piston, forward barrel? Both systems have their pluses and minuses.

[@WaterZer0]

Okay, there are legit advantages for the Stoner system, but this is some extreme copium. Keeping all the gas and fouling concentrated in one area makes cleaning much easier.

[@03redrubi]

@@WaterZer0 Maybe, but all the heat, and weight, IS concentrated near the muzzle. That's not to MY liking. Your mileage may vary!

[@WaterZer0]

@@03redrubi I understand the weight issue, but where else would you want the heat?

[@03redrubi]

@@WaterZer0 Not where my accessories, and hand are.

[@qedsteve]

ARRRGGGGG - and I had to get on Brownell's BRN-4 wait list

[@hamzaimran7086]

Both of you are real geniuses

[@silent1967]

Now I'm wondering if I have a homemade icecream churn or a homemade icecream bucket !

[@sinisterthoughts2896]

As I already knew. Once you understand what direct impingement really means, and see how th AR works, it's self evident.

[@josephreisinger33]

As long as it works when you need it to, call it what you want. Happy holidays gentlemen.

[@sbreheny]

Good explanation. It would have been even clearer if you had a true direct impingement gun to show side-by-side (like you mentioned - the MAS 49/56 or the AG42/Hakeem/Rashid)

[@alfredjr2k556]

Both. Merry Christmas!

[@joshuabessire9169]

Can you do a segment on Canada's deadliest and most prolific .22 gauge fully semiautomatic assault rifle, the Buttmaster?

[@asherdie]

Videos like this bring out the comments that prove society does not care to know proper language and the definition of words.

[@AirmanDan916]

Hard to argue with facts. Great video guys.

[@chrlsraines]

Interesting view! But the technical operation never lies…

[@VincitOmniaVeritas7]

I can almost hear the ARFCOM users angrily typing right now…

[@mostlypeacefulmisterputin]

Direct impingement is this damn shoulder problem my surgeon hasn’t been able to fix since 2019😡

[@mikechandler236]

I like the BRM design better but it seems that the push rod could give out from heavy use and would need replacement

[@ThePatriotParadox]

Any good solutions for covering/protecting raw aluminum that's exposed, aluminum black? Anything else???

[@theshitheads3178]

Love you two! Wish there was a gun shop sitcom.

[@MavHunter20XX]

Those two holes are perfect for dropping oil in to get your gun back up and running.

[@hairydogstail]

They are also perfect for blowing out ingress that shuts down external piston rifles..

[@oloflarsson7629]

You could argue that in the MAS and the AG-42, the bolt carrier is the piston, though rapped around the gas tube, rather than being inside the sleeve of the gasblock. So a "long stroke bolt carrier integrated spigot gas piston", while the AR-15 is a "short stroke bolt integrated gas piston".

[@arapahoetactical7749]

Eugene Stoner was an Aerospace Engineer by trade. You can't have unbalanced forces on aircraft and expect them to fly well. He felt the same way about firearms. He wanted all action/reaction forces to be on the bore axis. I've seen slow motion video's of AK's being shot and you can see the barrel flex down slightly as the piston is pushed out of the cylinder about an inch above the bore axis.

[@hairydogstail]

Stoner was never an engineer..He never went to college..

[@gregelliott2537]

Ak vs Ar = apple's vs orange's It's an old and boring conversation

[@arlendavis]

I agree.

[@UnknownUser-fe5zu]

Rofl Steve 😂 “I just learned how to pronounce direct impingement”

[@madcratebuilder]

I've always thought of the AR as a piston system. Gas rings on the bolt/piston.

[@chipsterb4946]

Steve forgot to put his hip waders on for the level of BS Caleb was ready to sling on this topic… 😂 One important point: Eugene Stoner’s genius in designing that gas chamber inside the bolt carrier does something VERY important. The bolt is pushed *forward* while the bolt carrier is pushed backward. This relieves pressure on the bolt lugs, making it easier to disengage from the matching barrel extension lugs and reducing wear and tear.

[@ShortArmOfGod]

It gives the gas a chamber to expand into that allows it to impart force to the carrier, allowing it to move. The delay caused by the carrier moving and the bolt staying locked allows the chamber pressure to drop sufficiently, relieving pressure on the locking lugs. The lugs want pressure pushing them into the lugs on the extension, that's why they're called locking lugs.

[@chipsterb4946]

@@ShortArmOfGod I do not understand what point you are trying to make.

[@SammyTheSidePiece]

Is there a way to convert from DI to a long stroke piston system? Or do I need to swap the whole upper?

[@Chiller01]

So one must define the term piston. If one agrees that a piston is a disc travelling in a sealed sleeve from a high pressure area often created by hot gases to the low pressure area within the contained sleeve. That movement caused by the pressure gradient often drives an operating rod that creates the desired effect.There is no piston in the AR15. The hot gas fills an expansion chamber within the bolt carrier forcing the bolt carrier rearward.there is some slight movement between the bolt and carrier as it unlocks but that in no way is the classical sealed disc travelling across a pressure gradient driving an op rod. There is no piston like structure in the AR 15’s operating system. The fact that the gas exerts it pressure within the bolt carrier makes no difference. The high pressure gas directly forces the bolt carrier to the rear. The bolt is just dragged along as the camming action unlocks it so it too is forced to the rear.

[@LRRPFco52]

The bolt tail with gas rings is the piston. Desert Eagle and Mini-14 both have static pistons. AR-10/15 piston just happens to be an integral component to the bolt, so after the tail is finished its duties as a piston, it cams and retracts along with the carrier once telescoped.

[@rickstrandberg6398]

It seems like semantics though bolt has rings ( like a piston) but it seems like 2 actions or that the bolt is locking and unlocking while the bolt carrier blows back

[@elwayb.602]

That was one that was super informative for me at least I’m in the middle of deciding a piston system to go with like the hk416 but I also love the long stroke piston system that PWS uses does anyone have an opinion on primary weapons systems

[@doghousedon1]

I have PWS's piston guns in both 5.56, 308, and 6.5 CM. For me, pistons are more fun to shoot. I'm not sure why. Perhaps it's how a piston changes the recoil impulse. The PWS guns are every bit as accurate and reliable as DIs. The only downside is that they tend to be more expensive (around twice as). They DO run cleaner and cooler than DIs. But after all is said and done, I still hesitate to say which is best. They are different. Only you can decide which to get, which fits your needs and wants better. I'll go out on a limb for you. If this is your first AR, go DI. Give the 223 Wylde clambering a serious look see. Good luck.

[@Hogmagundy]

Nomenclature has accepted the terms for simplicity of differentiating the types of piston or "gas guns".

[@danielmackormack8524]

I would like to know if the Wolf short stroke gas piston 556 upper is a copy of the HK or SIG short stroke uppers . Do you know?

[@sanglee5100]

Shout out to Caleb’s barber for keeping that man looking fresh AF

[@waruikoneko1635]

Loved this episode. This is actually a topic of many a heated debate as I've tried to explain that the AR platform is both 'a little bit' piston, and 'a little bit' direct impingement.

[@asherdie]

It is nothing direct impingement

[@nickloven6728]

Internal piston via gas impingement?

[@waruikoneko1635]

@@michaelhill6451 Except, the means in which the AR15 cycles is the very definition of DI. The only piston action is merely to unlock the bolt by pushing it forward. Some of the gas is vented off, but much of it is used to push the bcg rearward. This is why you get gas blown out of the gaps in the rear of the receiver near the charging handle. Using an earlier prototype that never made it into production and doesn't represent the final design as an example doesn't work, my friend.

[@waruikoneko1635]

@@michaelhill6451 And the debate continues. I guess I'll have to do some reading!

[@asherdie]

@@waruikoneko1635 you are not understanding the physics and saying Eugene didn't know better than you on his own design????? You got a wall of participation trophies, don't you.

[@BaconSlayer69]

Internal piston system to be specific

[@nunyabinness3574]

Steve had his cloaking ability activated at the beginning...sorry steve, didnt see you there! 🤣

[@PollyMercocet]

It’s actually a internal piston driven system that we somehow started calling direct impingement

[@hangnwithdosei3266]

IDI... Improved Direct Impingement The gas is still directly impinging (impinging: the act or fact of striking or touching something, or the effect produced by this) upon the bolt carrier group. It is just doing so in a way that is greatly improved over the methods of direct impingement utilized by previous designs.

[@WaterZer0]

Serbu BFG50A is also DI.

[@coreymoyers]

Three fun facts for everyone: 1) Everyone wants to be right all of the time. 2) Many people are stupid, but none of them realize it. 3) A P320 will not fire without the trigger moving rearward.

[@davidasbridge6525]

I hope you respond because I am curious if I would be correct in saying it is a gas impingement piston system.?.?.?

[@TeranRealtor]

I love these guys and have learned a lot from them..... but today's talk was only good, for the coffee time together.

[@keyhole9601]

Nice theory.... But mechanically speaking, it's just an expansion chamber, instead of a pistons.

[@AR15andGOD]

no because it reciprocates according to the gas pressure which is a piston

[@keyhole9601]

@@AR15andGOD nope, it's as an expansion chamber, not a piston, just to unlock the bolt. An "apparent piston".

[@zorngottes1778]

I agree!!!!

[@KittehONaLeash]

i like the term "in-line piston system"

[@Nickrioblanco1]

I should be call the "Stoner Operating System". I think he deserves that honor.

[@samvittoria9692]

Steve pulls out the SOLGW BCG. Nice.

[@allthingsconsidered3211]

Can we talk about carrier tilt

[@johnw4999]

Maybe a direct impiston system? I just made that up, lol Seriously though, you are correct and gave the best explanation to date. Unless your running a non-transferable FA AR, have Joe’s $1.7T budget for ammo, and shoot everyday, a “direct impingement” system will serve you very well. The piston gun is cool and all, but as we add accessories to the platform, you’re better off loosing weight where you can, and carry more ammo - never run black on ammo! Also, as you age not so well like this ole retired combat gruntpa (ABN), you tend to gravitate to lighter systems, lol.

[@allthingsconsidered3211]

Can you measure, slow mo, the difference between di vs piston gas blowback gasses

[@opencarry3860]

Come on, we all know the dohicky moves forwards and back inside the thingamajig when the trigger is pulled. These guys are just making stuff up.

[@ronniemaeker9102]

Justaquestion... Since gas compresses...is the shorter amount of time the gas is in the piston gas block relate to a quicker reaction time of the bolt...? Does the compression of the gas in the tube come into play with the timing of the operation...?

[@armynurseboy]

The AR gas system is tightly balanced and timed. It requires a set amount of gas energizing the gas tube and expansion chamber for a specific amount of time and specific pressure. You affect that by gas tube length, gas port size and dwell time. This is why you can't simply chop barrel length without making adjustments, else you start introducing timing errors.

[@rifleman-sw2go]

Steve - you miss spoke just a little bit at around 4.58 in the video. As I understand it, the bolt is locked in against the chamber because of the lugs. So therefore the bolt doesn't go forward - the carrier gets pushed rearward from the gas. For people who understand the gas system, we know what you meant. For people who are trying to understand the gas system, what you said might be confusing. I enjoy you guys videos, so keep up putting them out. I've learned a lot from you guys and others like you. John

[@tedhodge4830]

The forces from the gas still expand in all directions including towards the bolt when it is in battery. The force from the gas expansion has to be overcome before the bolt can move rearwords, I think that's what they meant.

[@redtra236]

I would call it direct impingement because the gas is still interfacing with the bolt carrier in the action of the firearm, where with what is conventionally referred to as a short or long stroke piston the exchange happens outside of the action of the firearm. But I can see both sides.

[@tonykees9044]

Nice