Nice BB!

Very nice, I've never seen it layed out so simple and so well explained. I'll definitely be keeping a link for reference.

Good post.

Nice write up and well illustrated.

this should be Locked & Tacked


thank You BB!

nice and easy to understand for the newer people, good job on the pictures and comparisons.

Well Done, BB! A well put together piece on Eugene Stoner's unique, simple and briliant design on the blending of two completely diffrent systems, gas piston with operating rod, and true direct gas impingement.
   
Thank you for your time, BB!

Overall very informative.

However, I'm not so sure I like this part 100%.



The Ar15 does have rails.  Look at the top&bottom sides of the bolt carrier.

Also notice the top of the upper reciever and bolt carrier.  A groove in the upper, and a part sticking up off the top of the bolt carrier.  Though it's not a rail in the sense of an M1 or AK, it nonetheless indexes the bolt carrier in the upper.  Without it, the bolt could rotate freely and do a good job of not working.


One other thing, the AR's gas system does push the bolt off center–– the BC is pushed down and rearward by gas.  I'm noticing this on my new Del-ton rifle that the finish wear is uneven on the 'rails' on the bolt carrier.  The front area of the top rails and the rear area of the bottom rails have the finish wearing faster than the other areas.

Very nicely done and certainly needed around here. I admit it took me a year or two of being an end user before I realized exactly how everything worked in my M-4.

yak

Thanks bigbore. This was pretty informative for me. It sounds like it would be a good read for those considering retro-fit pistons systems before they purchase.

Quoted: With all the hype being swallowed about gas piston conversions, I’m convinced most folks don’t understand how the AR works as it left the box

Good info, with nice pics and descriptions too. I must admit though, it appears to be yet another one of your piston bashing posts in disguise which is evident in your very first sentence.

Just because you have issue with and dont believe an ARs action can be unlocked with a piston doesn't mean that it's true. Is this your attempt at another gas piston bashing thread disguised as an informative thread? Maybe a little damage control after your other piston bashing thread?


We all know understand you have a problem in general with gas piston rifles and conversions. It is your belief that the AR was not designed to be unlocked by a piston striking the carrier key, we get it, we get it. Dont tell me you're talking about the conversions and not the complete rifles, Im not buying it. The basic principals of the two are the same.

While I like the current DI system and dont find fault with it, i do find your persistent criticism of piston conversions and your crusade to prove the AR cannot be unlocked using a piston to be both annoying and unsubstantiated (i.e without scientific facts).

In speaking with technical people at LWRC, Barrett, and LMT, and sharing your exact thoughts, they seem to have a different opinion than you with regard to this. There are also many dealers on this board who are selling gas piston rifles and conversions and I find that your persistent threads to sway people away from them because you dont approve of them is doing a great disservice to the dealers on the board who are trying to make a living and sell piston rifles and conversions as well as the people who are satisfied with their piston conversions. As a dealer yourself I'm surprised you cant see this.  

"Also – don’t buy into the bullshit about NM carriers being a tighter fit enhancing accuracy, they don’t. The accuracy comes from the consistency of the bolt lock up mentioned above – which has nothing to do with the carrier. "

Got my attention, So what would you recomend to be the best Bolt out there right now as far as accuracy?

we need one of these everyone in a while.

the last one lacked pics, so this is probably easier to swallow for the uninitiated.

this is IMO the coolest part of the AR ... what makes the design team legendary in my mind:



ETA:
for the first two years of my gun life (04-06 I know I'm a newb!) I thought the gas impacted the carrier key and pushed it back there.

Never really could figure out where the gas went after that or why it didn't blow out the ejection port.

when I read my quoted part above (in the previous post on this about two years ago), I was floored

Thank you BigBore.

Only quibble:

I wouldn't say that the "gas" is the operating rod that rotates the bolt cam and unlock the bolt, if that is how you define an operating rod.  Rather, the bolt carrier is the "operating rod" that rotates the cam.

If, on the other hand, you define the operating rod as the piece that transfers force to the bolt carrier, then yes, the "gas" is the operating rod.

Perhaps the "piston" AR should be named the "Rodded" AR :)

Another thing, I agree with LedZepellin above that the AR bolt carrier does ride a "rail" inside the receiver.  In fact, you can say that the entire upper receiver is the "rail".

If we can have DI without the bolt carrier making so much contact w/ the inside of the upper receiver, that would improve AR reliability under adverse weather conditions.

you hosed a carrier for this?

was it any harder to mill  than average materials?

So since someone is trying to sell it, no one should be critical? While I respect the dealers desire to make money, it doesn't make the "piston" conversions immune from criticism.

I dont believe anything should be immune from criticism so long as the criticism is substantiated (i.e. information from a reliable source or from testing done in a scientific manner).

Great stuff...

I'm keeping all these cutaway images as references on my computer.

What are the image rights on these?

Nice writeup, bigbore!

The only thing that's not clear is the following:



Upon firing gas is vented down the gas tube and into the back of the carrier, aft of the rings on the bolt tail.  You say that this gas pushes forward on the bolt (assisting unlocking) and at the same time as it pushes rearward on the carrier.  The carrier then moves rearward, camming the cam pin (natch!) unlocking the bolt, and allowing the bolt to be retracted from the barrel extension to complete the extraction and ejection sequence.

What's unclear to me is just how much forward movement the bolt is under.  To my way of thinking the bolt is pretty much as far forward as it can go when it's locked; the vented gas acts (if at all) on the bolt to maintain a good seal until pressure drops, and the bolt is cammed to unlocked and extracted from the barrel extension as the gas pushes the carrier rearward.  

So, is this forward travel of the bolt real?  If so, just how much movement are we talking about?  Or is it really just a "pressure" effect with no measurable forward travel of the bolt?

I know very little of the AR design as my first one was just completed 2 weeks ago, What makes the piston better than the DI system? Could I see the information on that?

A few thousandths of an inch maybe.  Take your bolt out of the carrier, then put it in the upper and rotate the lugs so that it's locked, then move the bolt forward and backward.  There is a slight gap between the barrel face and the lugs.  On a loaded cartridge, the bolt headspaces and the lugs are mated to the barrel extension pretty close.  How much it moves forward on a fired case, I don't know.. not much, but probably enough to ease the rotation of the bolt.



As for DI vs. piston uppers..  The piston uppers I would think have a slight reliability advantage.  Carbon residue isn't injected directly back into the bolt/action area, and is instead drained somewhere above the barrel.

I know for sure that my M1 garand is a hell of a lot easier to clean than my AR15.  Because all the bolt and receiver require is a wipe-down from an oily rag.  Then just soak the gas piston and cylinder in powder solvent and wipe them out.  

The AR requires cleaning out the inside of the bolt carrier (not the easies thing to get into), the bolt  (stuff is like plated on, almost has to be scraped off) and in the barrel extension lug area.

Now how long would it take for a DI system to throw enough carbon into the works to make it stop working?  I'd say well over 1000 rounds without a cleaning.  How long with a piston system?  probably a lot more than from a DI system.  Then you get into the whole mess of dirt/dust/water/mud etc.. in the gas system, where I believe the piston systems work better.

Given the choice I'd take a piston rig just for ease of cleaning in the receiver area.  Realistically, I'd expect to run out of ammo before I'd worry about either system failing on me.

Thanks, Zep...

I thought about this for a second or two when I read it, but BigBore is correct.  What he is trying to point out is a simple fact of physics - a gas exerts pressure equally in all directions.  So, yes, the gas pushes back against the bolt carrier, and it pushes forward against the piston-ring flange of the bolt - and for that matter, it pushes outwards to the sides, top, and bottom of the gas chamber in the bolt carrier.  The bolt is bound to move forward a little, and it's movement is obviously limited by contact with the barrel extension.  This part of the explanation is strictly correct, although not absolutely necessary to understanding the operation that takes place.

I, too, originally thought that the Stoner system used gas directed down the gas tube to "blow" the bolt back.  However, in reading and studying diagrams and pictures, I figured out how it works some years ago.  But I am an engineer by training, and so think about this stuff in a critical manner.

I sort of cringe, however, when I see the Stoner system described a "direct impingement".  It's really not "impingement" at all, in my mind, in the sense that the gas "impinges" on anything - it's actually just channeled into an internally-expanding piston, as BigBore has ably illustrated.

For many, many years, however, I was counted among the Stoner rifle's detractors - but I am of an age to remember the difficulties experienced in VietNam back in the '60's and '70's.  However, after talking to more recent veterans of the Army and Marine Corps, and inquiring of them their thoughts on the reliability of the rifle, I finally saw the light.  As a simple, lightweight, and robust design, the Stoner system has a lot of merit.

I have quite a bit of experience with other rifle systems, owning two M1's, an FAL, and a CETME, and they all have their pluses and minuses.  Ask me sometime what a bent (accidentally dropped) gas piston will do to an FAL's reliability.  There was also a long history of op-rod modifications to the M1 before they totally solved breaking problems - and bending the M1 op-rod to the exact degree to make a reliable rifle is an art, not a science.  You want to see a rifle that runs dirty, check out a CETME/HK-G3/HK-91 system with its fluted chamber.  I call my CETME "El Cochino".  Ask someone who speaks Spanish what that means.

Excellent post!

I second the motion,, all in favor,,, say "I"

to aid in illustration

Piston uppers vary quite a lot in terms of weak points and design flaws; there are also carrier tilt solutions that prevent the buffer tube wear that Bigbore pointed out.  Some have problems and need something like that, and some dont.  What this means is that these systems vary a lot, and there are no doubt some out there that would have been better off sticking with DI. (particularly if they could take the money they saved and spend it on ammo and training).

Bigbore spends a lot of his own time helping people out and providing information.  I don't think he really cares what people are "satisfied with"; from what I've seen, he's about what works, and that's something we could use more of around here.  I don't always agree with him, but I see no reason to dump all over his thread and accuse him of some sort of shadiness.  If he wants to make a specifically piston-bashing thread, then I'd expect him not to beat around the bush.

I can find no flaw in my LWRC operating system that cant be cured with their one piece carrier.  I dont even have to remove my bolt from my carrier to clean my BCG.  Gas coming down the tube and being diverted is not free from the effect of physics.  There is nothing about a DI system that will help accuracy vs my LWRC.  The unlocking and ejecting has no effect on a rifles whose bullet has already left the bore.  The accuracy is all in the barrel and bolt.  I dont see wear that is unusual in my LWRC.  It may be over priced for what it does but it gives me some advantages over DI and no drawbacks.  Its up to the end user to determine if that amount of money is worth the small improvements.  I would not buy a conversion as they exist from many companies.  I dont trust the quality and engineering.  I do like a cleaner gun, less blow back via gas not escaping the interface of the carier and gas tube, and total lack of port noise near my ear.

wow

First off, I likes cutaways!

Anyhow, I didn't realize how many people (using percentages of thsoe who responded) didn't know that the gas pressure pushes the bolt forward (hence the gas rings).  That said, I will comment that a previous poster was right on teh AR ahving a "rail" for teh carrier to ride in.  The gas key in the charging handle no doubt aids in alignment.

Bump so more people can have this info

why is this not tacked?

Tag for when it's not zero-dark-thirty so I can actually comprehend things.

wouldn't this thread eventually expire, or drop off into the wherever it is that old threads go unless it keeps getting bumped or tacked?

Looks like it's preserved forever in a reference item link:



http://www.ar15.com/content/page.html?id=535





 

what exactly is the point of this post??? i understand that definitions of "piston" and "cylinder" are up for debate.... but this post really is about the "anti-piston" movement that has been going around..... This post really seems to explain the internal mechanics of an AR, but does not seem to take into account any of the physics of a "gas piston operated" AR...... I think that if one is trying to say that a certain object is better than another, both of them need to be thoroughly explained...  I honestly do not see where this post has any solid information for or against the newer style uppers.

This simply looks like a hate tread of the new technology.... reminds me of the debate of a carburetor vs. fuel injection... people just wanting their opinion to be heard regardless of the reasoning behind it...

Fantastic job. Thanks!

There are drawbacks, you just refuse to see them

Great explanation, thanks.

This should be sticky forever. It will help a lot of people understand how the AR15 action works.

What about special 'billet' upper receivers like the MUR or stealth. How would the receiver affect accuracy?