DI or Piston?

[tom12.7]

It was put out at a time when the DI 10.3-10.5" guns were over gassed by a bit and had issues running cans. The same isn't true overall today.
Without a can, the HK does eject more particulate/ fouling, but at a cost when looking at function. There's stresses/ strains/ deflections that come into play, there's also issue with some timing of events that don't help operation.
With a can, both systems unlock and open before the barrel pressure reduces to 1 bar absolute. That means fouling does blow back into the receiver. The time it takes to relieve reversion and release residual pressure (that contains fouling) is a longer event than the unlocking and opening by a fair portion.
I have experiance with the HK 416 series in various evolutions with the 10.x" and 14.5", including usage with AAC, Surefire, and KAC cans.
At this time, I don't see the need for this for most users.

[Koshinn]

Some piston gas blocks on ~10.5" guns can't be disassembled if certain suppressor mounts are on the front of the barrel.

With a AAC 51T mount for example, you can't actually remove the piston from some front-removal plug/blocks if it's too close to the gas block itself.

I'm guessing that's why the HK, at least, disassembles from under the handguard. Probably LWRC's reasoning too.

[tom12.7]

Removing the rail does't help out with maintaining zero with many rail mounted accessories. It does take time to function check those. That's another item that could be considered less than optimal for that.

[Arctic1]

You don't have to clean the piston that often to make that an issue.

I am aware that adding a suppressor or a BFA will lead to more fouling inside the receivers - stated that earlier in the thread.

When compared to the C8 SFW, the HK416 exhibits longer parts life across the entire system.

Which costs in terms of function, stresses/strains/deflections and timing issues are you referring to?

[tom12.7]

I'm not talking about cleaning the piston.
Agreed, that a can or BFA will induce more fouling back into the receiver for either.
Does the 416 exhibit a longer parts life? For some, it could, but the question within that is the with the same material set. If that same was applied to a DI model, I would have to question some things, to compare them the same way. Sure, some aren't as needed as others in details for that application, but what point are you trying to end to?

[Clint]

Not wanting to jump in the middle, but only to point out there may be theory to explain all observed phenomena.

Comments heavily edited to focus on technical items.

Let's see if this works...

It becomes easy to understand, for example, that the gas holds more heat at the gas block and how it cools as it travels through a tube.
It becomes obvious that the face of a small diameter piston exposed to gas with more heat is going to get hotter than the face a larger diameter piston exposed to gas holding less heat.





Pistons still get heated and coated with fouling whether they're part of a long stroke or short stroke system, or if the piston is in the carrier or the gas block.
Lube helps keep fouling moist and soft.
Heat causes fouling to dry and harden.

There is less heating of the bolt, bolt carrier and associated parts on a short stroke piston gun than there is on a DI gun. That is fact, and has been measured using heat imaging. I'll try to dig up the images later today.

Barrel, chamber and gas block temps run about the same, depending on barrel profile. I'll try to dig up the images later today.

It seems quite clear the gas block on both examples gets quite hot.

Here are very round numbers for temperature from the SCAR comparison after 112 rounds of rapid semi auto fire.

Chamber / barrel 250
Gas block 450

DI gas key / tube 240
DI carrier bore 190
DI bolt body 160

Ambient 75

In the HK and FN designs, the gas block and piston are at similar high temperatures.

In the DI design, the carrier and bolt (chamber and piston) are significantly lower.

On the 416, the piston gets hot, while the DI system gets about half as hot.


The relative temperatures are generally agreed upon.

The disagreement/misunderstanding seems to center around the mechanism of fouling as it relates to heat.

The gas vents out under the handguard as the piston is pushed rearwards by the gas pressure. Some gas escapes there. Most of it is ejected out of the port on the gas block.

There isn't much fouling on the piston itself, most of it is just discoloration.

This is 416 specific.

Most of my experience is with the HK guns, and there is next to no fouling in the gas block, the piston runs very clean although minor build up can occur near the tip. It is likely that there will be differences between the designs.

To be clear, I didn't claim the carrier and the bolt of a standard AR is as cool as an AR with a gas block piston.
What I said was that the gas is carrying less heat when it gets to the carrier because it dumps heat as it passes through the gas tube.

When the piston is relocated to the gas block, the gas has more heat, the surface of the piston face is smaller and it's inside the gas block which gets much hotter than this carrier does.
The piston in the gas block gets hotter than the piston in the carrier.

...

When the piston is in the gas block, that's where the rifle will dump most of its fouling.

Hard carbon will build up the face of the piston. Hard carbon will build up on the piston of a standard AR. A standard AR piston has the advantage of being easy to lube. Lube helps to keep the fouling soft.

On the HK design, there is no room inside the gas block for carbon to be deposited.
Most of the hot gases are vented out of the front of the gasblock.
Some gas escapes out of the rear of the gas block, as the piston moves rearward to push the piston rod back in order to unlock the BCG.
This manifests as carbon build-up on the inside of the handguard in the area around the gas block.
This has no bearing on the difficulty or ease of cleaning the weapon.

The piston on the HK is designed to withstand the heat that it is exposed to in the gas block. It is also supposed to be lubed.
The only thing that wears on the piston are the gas rings. It gets discolored from the hot gases, and a minimal amount of carbon can build up on it, towards the tip. Cleaning the piston is a 1 minute job, at worst.



This is from my old duty gun. This picture shows the piston the gun came with on the right, and a new one on the left. The discoloration is clearly visible. It had approx 10k on it. The only place where any carbon is visible, to the extent where you will get blackened fingers, is from the top ridge and down - ie the part of the piston that is not inside the gas block.

Direct observation shows a very hot piston with minimal fouling.

"Conventional wisdom" says Heat = Fouling = Bad.

What can explain this observation?

My reason for pointing out that the piston in the gas block is exposed to a higher level of heat is to counter the heat criticism leveled against the direct gas system.
Critics of the direct gas system claim that directing hot gas against the piston in the carrier is a design flaw.
Yet, they fail to realize that a piston in a gas block is not only exposed to hotter gasses, but also has less surface to deal with the heat and gets hotter.
If a piston is more than capable of handling the heat in a gas block, it can handle the reduced heat found inside a carrier

It means the piston has to deal with at least as much fouling when it's in the gas block as it does when it's in the carrier.
The gas carries particulates. The particulates are what I'm talking about when a talk about fouling.
As the gas travels along it's merry way, it leaves deposits of fouling behind, especially when the gas needs to change directions and when the gas cools.

About the gasses cooling in the gas tube.
Heat transfers from the gas to the tube then radiates from the tube to surrounding air.
That means the gas carries less heat as it enters the expansion chamber of the carrier compared to when it enters the gas block.

There is the kicker in bold.

Fouling deposits when gasses cool.

How many people have had to clean out the gas tube in a DI AR?

Why is there minimal buildup in the gas tube?

Why is there minimal buildup on the inner bore surfaces of a flash hider?


My theory on the fouling mechanism is: When there is plenty of energy, both thermal and mechanical, the particulates stay suspended and moving.

When the energy drops due to cooling or performing work, the particulates deposit and stick.

This is similar to how moisture condenses on a cold glass but not a hot one or how a river deposits sediment only when it turns and slows down.

As to your point on the gas tube heating up and slightly cooling the gas, sure.
From the gas block to the carrier key you lose about 200 or so degrees on the DI gun, compared to 370 on the piston gun.
The DI bolt body is 100 degrees hotter than that of the SCAR or the piston AR. The carrier key is 160 degrees hotter.
The firing pin and cam pin are 120 and 90 degrees hotter, respectively. That is a significant difference.

Per HK, their design is so that 90-95% of all the carbon/particulate matter is ejected from the from of the weapon - out of the muzzle or gas block.

Perhaps the "issue" with the DI design isn't that the bolt gets hot and therefore fouled.

It may be the bolt doesn't get hot enough to prevent fouling under normal use.

Think about that one for a minute.


This may explain why the DI AR usually fails under "meltdown" conditions only by blowing either the gas tube or barrel, NOT by excess fouling.

Keep in mind, this is ~1000 rounds with no lube and components get very very hot.

However, a multi day 1000+ round carbine class generally will cause fouling related failures unless lube is added once or twice a day.

Much lower temperatures at work in that scenario and more fouling related issues.

[Spin Drift]

Using your river example, the higher the velocity, the more "material" in suspension, whether river silt or fouling, seems applicable.

Would one want the bolt to receive/experience sufficient heat to prevent fouling? Honest question.

The answer is how do you use the rifle? Personally, few scenarios have me getting my rifle so hot as to fail.

My beater SR-15 often goes 3-4k between any sort of cleaning. That's enough for my purposes.

Quality, clean burning ammo has got to be a factor.

[MegademiC]

This is good stuff, but it seems were back to square one.

If you can't lube between outungs, or you need to dump over 1000 rds full auto at one time, you should be looking at something other than an ar.... more like a 249, rpk or something?

At which point the cost is irrelevant.

[vicious_cb]

I'm not talking about cleaning the piston.
Agreed, that a can or BFA will induce more fouling back into the receiver for either.
Does the 416 exhibit a longer parts life? For some, it could, but the question within that is the with the same material set. If that same was applied to a DI model, I would have to question some things, to compare them the same way. Sure, some aren't as needed as others in details for that application, but what point are you trying to end to?

HK uses all proprietary steels. A 416 bolt costs $330, thats more than $100 more than a KAC E3 bolt that uses aermet steel and those things never break. God only knows what kind of steel they use for their barrels.

The M4 TDP is literally in the stone age compared to whats available in modern manufacturing today.

[MistWolf]

Why is there minimal buildup in the gas tube?

Why is there minimal buildup on the inner bore surfaces of a flash hider?

My theory on the fouling mechanism is: When there is plenty of energy, both thermal and mechanical, the particulates stay suspended and moving.

When the energy drops due to cooling or performing work, the particulates deposit and stick.

This is similar to how moisture condenses on a cold glass but not a hot one or how a river deposits sediment only when it turns and slows down.

Perhaps the "issue" with the DI design isn't that the bolt gets hot and therefore fouled.

It may be the bolt doesn't get hot enough to prevent fouling under normal use

Good points.Gas velocity has a lot to do with fouling build up. Fast moving gasses reduce build up by scrubbing out some of the previous deposits. That's what keeps the gas tube clear. It's when the pressure and velocity tail off that most of the particulates drop out to stick to surfaces. That's why we see the most build up where the gasses are vented. That's why we see build up where the flow is cooled, slowed, interrupted or encounters turbulence