It seems like articles often make differentiations between the two.

I looked at an archived thread on THR and it said that one of the reasons the Mas-49 was known for it's reliability and the M-16 was known for issues early on because the Mas-49 is tilting bolt and deposits gas on a non-essential part of the bolt and the M-16 deposits it on the rotating lugs, which caused powder build up issues with the early ammo. Does that seem about right?

Someone will be along that can explain it better than me, but my understanding is that the Stoner system is not a pure DI system and uses a piston located in the bolt carrier.  

From Armalite's tech notes.



http://www.ar15.com/archive/topic.html?b=3&f=126&t=502822

Bolts are bolts, and bolt carriers are bolt carriers, not pistons.
I find it humorous that the "it's not really DI, it's xxxx piston" line seems to be a recent thing, coinciding with the popularity of true piston designs the past few years.
Nothing like a little re packaging to make your widget cutting edge all over.

That's not really an accurate assessment.  The M16 has always used direct impingement in combination with a an expansion chamber in the carrier.  The capturing of that gas in a ring sealed expansion chamber means a lot less gas is needed to push the carrier to the rear.  Once it's moving, the momentum of the carrier cams and unlocks the bolt and cycles the action.

The only functional difference between a piston operated system - like the AR-18 - and the M16 is where the "piston" is located - and a piston is really what we're talking about when the carrier is moved back by gas in a ring sealed expansion chamber, it's just a case of having a fixed piston with a moving cylinder.  

In an AR-18, FAL, AK, SKS, etc the piston is located at/near the gas block.  On the M16 it just happens to be located in the bolt carrier itself.  Both are in fact piston driven, it's just the location of the piston that led the M16 to being referred to as a "direct impingement" design.  It's arguably a hybrid as it incorporates a piston as well as "direct impingement" given the routing of gas to the bolt carrier.

Personally, I think most of the piston "hype" has passed and many of the piston driven variants of the AR-15 design leave a lot to be desired.      

----

That's should also address the difference between the M16 and other direct impingement designs - it's a piston system versus simple gas (and lots more of it) blasting against the carrier.

No, it's not a recent thing.

OOOHHH, "true" piston designs................

The repackaging being done is by those who have the facts staring them in the face about what a DI really is (and it ain't the Stoner system) and they can't accept it because they have been using the wrong terminology for so many years.

And external pistons, what you call "true piston designs" have been around long before either of us was born.

Even though this clearly explains the difference and even names the guns that really are DI, some will just call it "repackaging" because they can't accept the facts.

I'm not too familiar with each of these rifles, but your posts are very confusing.  What are you trying to say?  That newer "piston" ARs (like an HK416 for example) have no technological advantage over a normal AR (like a Colt M4)???  Or that both are piston systems and a normal AR (e.g. Colt M4) is not really DI???

(I am trying to learn, not argue.)

I am not trying to say anything, just showing that the Stoner system is NOT DI.

Both are piston systems (one external and one internal) and a normal AR is not DI, never has been.

Nothing confusing about that.  I don't see how you think I claimed one is better than the other.

I have my thoughts on that, but I did not state them in the post you are responding to nor in this thread at all.

Other rifles - gas from gas tube pushes against bolt/bolt carrier and pushes it rearward.  Gas is just exhausted into the receiver.  This is how the uninformed think the AR15 works, ie, the "shits where it eats" slurs.

AR15 - gas from gas tube enters into expansion chamber/cylinder, pushing bolt forward and bolt carrier rearward.  Gas is exhausted out exhaust vents from the cylinder out the ejection port.  Gas is contained within a system to make it work, not just allowed to blast out of a bare ended tube.  In a way it's similar to an external combustion engine, like a steam engine.

In the Stoner system the forces act on the axis of the barrel bore. The bolt and bolt carrier expansion chamber share the
same axis as the barrel. On a Hakim/ Ljungman the gas impinges at the top of the bolt carrier,
off-axis from the barrel and asymmetric to the bolt carrier center of gravity.

(OK, there's a change of direction in the gas as it goes into the bolt carrier, and that's asymmetric.)

I'll go ahead and speak the heresy here.

The AR-15 gas system is a pretty elegant design with enough reserve energy that it'll even work with worn and missing piston rings.  The principle objections to it are:

1) as noted above, the mistaken impression that it shits where it eats, and

2) the hot gas carries heat to the bolt carrier.

The first is incorrect as the small amount of gas involved is vented outside the action and the second is true enough, but the amount of heat transferred is small in comparison to what's transferred from the chamber, locking lugs and case head in almost any design.    

The traditional piston design is however not perfect.  They are more complex, proper alignment of the parts is critical and they have more points subject to wear, gas erosion and corrosion.   In that regard, and owning several examples of traditional piston operated semi-auto rifles, I'd argue the AR-15 system is a minimalist system with less that can go wrong, and what can go wrong is easier to diagnose and fix.

Gas directly impinges on the carrier = direct impingement.

No amount of wordsmithing i.e. "expands," "exhaust," "cylinder," etc. changes the fact that gas directly contacts (impinges) the bolt carrier.  There is no piston.  The expansion chamber may perform the function that a piston / operating rod would otherwise perform, but that doesn't make it a piston.  The fact that the M16 uses a more efficient means of DI than earlier models does not change that fact that it is still DI.

This.

Leave it ARFCOM to re-invent AR-15 terminology based on semantics.

Internal piston and external piston...

Like it's taken several decades to come up with a proper description for the method of operation

My car engine has a cylinder with a piston.
My air compressor has a cylinder with a piston.
My soap dispenser has a cylinder with a piston.
My grease gun has a cylinder with a piston.
A steam engine has a cylinder with a piston.
A syringe has a cylinder with a piston.
A shock absorber has a cylinder with a piston.

My AR has a direct impingement.  Got it.

Technically, it has always been a bolt with gas rings.

Since we are talking about the gas system, the AR is most assuredly a DI system. What happens after the gas enters the carrier is what differentiates the AR and other DI systems (MAS-49, etc...). A piston-operated gas system refers only to the method that mechanical energy is transferred from propellant gasses to the bolt. Whether that bolt is a tilting bolt, rotating, or whatever else is immaterial. When comparing a piston to a DI AR, the receiver parts are the same (mostly); it's the parts up front that are the focus of discussion.

Gas rings...  Kinda like the pistons on my car having piston rings.

ETA:
Even the DI Ljungman Has a piston.  It's labeled #12 in the drawing below:



The main semantic issue I see here is all gas operated rifles have a piston.  Without a piston it couldn't generate the force to operate the rifle.  However, the issue is, "Where is that piston located?".  If the piston is far away from the gas port in the barrel folks like to call it direct impingement.  If the piston is close to the gas port they call it piston operated.  They are, in fact, both operated by pistons.  One just transfers the mechanical energy the piston generates to the bolt carrier using a rod and the other takes the gas to the piston located near, in, or on the bolt carrier using a gas tube.

I'm going to propose the AK 47 is a direct impingement system.  The piston is part of the bolt carrier. The only difference from the Ljungman is the gas tube is really short and the piston is extra long.

No.

The gas never enters the carrier (or anything) on a true DI rifle, that's the whole point of the distinction.  In a true DI rifle, the gas is  uncontained.  In an AR15, the gas is contained within a system which extracts energy from the expanding gas in a controlled fashion and then the gas is exhausted.

I was just playing with my MAS-49/56's and gas vented from the barrel most definitely enters the carrier and applies force to the carrier in order to unlock the bolt. The gas must be contained within the carrier momentarily for energy to be imparted so the carrier can move. In both the MAS and the AR, energy from the gas must push the bolt carrier rearward to facilitate unlocking. The difference is that, while the carrier begins its rearward motion, the AR's bolt is first pushed forward and must rotate before it can be unlocked.