Gas piston rifle testing.

[ClickClickD'oh]

HK is a company with a reputation for excellent products.

There's another reputation HK has around the community...

...there's a very good chance that there wasn't any water in either the HK or Colt barrels.

Then the colt failed because of mystic garden gnomes?

 

[Maverick223]

Then the colt failed because of mystic garden gnomes?

Definately a Gnome...or maybe the illusive Troll...because it couldn't have been caused by water in the gas tube.

And FWIW I think that the test was "somewhat" rigged...I do NOT think that the "Competitor's Rifle" was tampered with in any way or that the video was cut to hide something...but I still maintain that the HK is a better rifle...and their pistol is the $h!+.

 

[ClickClickD'oh]

...because it couldn't have been caused by water in the gas tube.

Which do you think is stronger, the reciever or the gas tube?

 

[MCMXI]

Then the colt failed because of mystic garden gnomes?

I don't presume to know for a FACT why the Colt failed and I certainly don't presume to know for a FACT that the test was rigged. As for HK's "other reputation" ... well, that's just more of the same isn't it.

I've already posted an image from the video showing the Colt coming out of the water butt first ... and very little water, if any, is shown coming out of either barrel just prior to the trigger being pulled.

If you think about, if HK wanted to guarantee failure of the Colt by mechanical means, why not simply shove an obstruction in the barrel of the Colt. No one in internet land would be any the wiser.

 

[ClickClickD'oh]

If you think about, if HK wanted to guarantee failure of the Colt by mechanical means, why not simply shove an obstruction in the barrel of the Colt. No one in internet land would be any the wiser.

Unpredictability

 

[Maverick223]

Which do you think is stronger, the reciever or the gas tube?

Very likely the gas tube; due to the smaller size when subjected to internal pressure (rather than flexure under heat where it would certainly be substantially weaker).

Unpredictability

Because the time that it takes for the water to come out before a shot is a certainty...and they timed each step to get it right prior to shooting. I feel that the obstruction would be a better defined way to cause a KB than water inside the bbl.

 

[RedLion]

Hmmmmm.... well I read this

That's why so many companies have been working on developing a cleaner M16/M4 design.

and this

I have become familiar enough with the Eugene Stoner design to describe the M16/AR-15/M4 firing mechanism as being a magnet for hot gases and burned gunpowder that can cause malfunctions and damage the bolt assembly.

and I'm wondering... Why would there be such a problem? The Army and how many other countries don't see it? and then I read this...

If you belong to a tactical team or a special operations unit that trains with M4s on a regular basis, I strongly suggest that you buy a gas piston rifle or carbine for your individual or agency use

Ohhhh! I get it now! looks like there's not enough ad space on your internet page/magazine article. Yea, If I could get a couple of bucks out of something like this I'd say it was the UP AND COMING NEW SPACE AGE SUPER COOL GOTTA HAVE IT. And then I remember the last time something like this happened.....

And nobody really cared. Why? because the AR, yes the DI AR-15, is more than good enough for what it was made to do. If its not the ammo, its the gun itself, but everyone know the price of fame is quite large.

 

[ClickClickD'oh]

Very likely the gas tube; due to the smaller size when subjected to internal pressure (rather than flexure under heat where it would certainly be substantially weaker).

That would be the same amount of pressuer that vents through the gas tube into the reciever and doesn't make it explode?

You can bend a gas tube quite easily with your bare hands into confusing shapes... not so much a reciever.

Because the time that it takes for the water to come out before a shot is a certainty...and they timed each step to get it right prior to shooting. I feel that the obstruction would be a better defined way to cause a KB than water inside the bbl.

Okay, back to logic Step 1.

You are going to tape a test of a product failing spectacularly do you:
A) Just use the product until it fails eventually at some unknown time in some unknown manner. (shoot the hell out of it)
B) Set up the product in such a manner that it is likely to fail, but still at a non determined time and in an unpredictable manner. (obstruction)
C) Dateline NBC

For those of you not familiar with the Dateline fiasco... NBC decided to do an investigation and claimed they had "proof" that GMC trucks were unsafe as they burst into flames alot. Of course, they had video proof of such. Only problem, NBC had rigged the "tests" to garner the results they wanted.

Of course, setting up the tests to generated the exact desired result just happens to be the most effective way to catch the result on film. Problem is, it almost always leaves inconsistancies that the experts tend to notice... like the wrong side of the reciever fracturing.

Long story short, if you want a "test" to go absolutely your way on film, you remove all the uncertanties... Cut the reciever, or put pyro under the gas tank.

HK pulled a Dateline. It's clear to see for anyone that has ever seen an AR/M16 pattern rifle have an explosive failure.

 

[Grantman]

HK pulled a Dateline. It's clear to see for anyone that has ever seen an AR/M16 pattern rifle have an explosive failure.

I'd be more inclined to believe HK than that speculation given the information at hand.

Problem is, it almost always leaves inconsistancies that the experts tend to notice... like the wrong side of the reciever fracturing.

Given that a company like HK would have experts coming out its ears that would put most commentators here to shame, 'the receiver fracturing on the wrong side' is far more indicative of a natural failure. (that's if it even did 'fracture on the wrong side'. Lets see someone else try it to prove 'it was on the wrong side')...

 

[ny32182]

I'm not commenting on the rest, but be assured that the gas tube can contain *far* greater pressure than the upper receiver if you want to compare PSI for PSI.

The pressure in the thick steel carrier that operates the action (around 1000-1500psi according to documentation I have seen) is going to be well under that which is present in the gas tube (~15k psi at the port, depending on gas system) as the gas pulse expands there. The upper receiver is going to be far weaker than the carrier, as it is made of weaker, thinner material.

 

[ClickClickD'oh]

Given that a company like HK would have experts coming out its ears that would put most commentators here to shame...

Because they're Teutonic right? That's why HKs "experts" would never stage a "test"?

I'm sure that's also why HKs "experts" most certainly know so much more about AR pattern rifles... especially when compared to lowly non-Teutonic schmuks who only carry the things around every day...

...receiver fracturing on the wrong side' is far more indicative of a natural failure

Wait... what?

The reciever fracturing in a way that never happens normally is more indicative of a natural failure than tampering to you?

Really? Something happening that never happens seems natural to you?

Really?

Honestly?

Think about that for a second.

Every other AR pattern overpreasure failure ever blows the left side of the reciever off... this one, that just happens to be filmed by HK, during an HK set up test for HK promotions just happens to blow on the other side... and that's natural to you?

Lets see someone else try it to prove 'it was on the wrong side

There are hundreds of reports out there about AR pattern rifle failures. If you talk to the right people you can even get copies of them to use as training aids in why it's not a good idea to drop your muzzle into the sand.

They have a certain pattern in them...

 

[kwelz]

So what I am hearing from Grantman is that only HK has experts and nobody else knows what they are talking about. Sounds about par for the course for your posts.

 

[Grantman]

Because they're Teutonic right? That's why HKs "experts" would never stage a "test"?

I'm sure that's also why HKs "experts" most certainly know so much more about AR pattern rifles... especially when compared to lowly non-Teutonic schmuks who only carry the things around every day...

Wait... what?

The reciever fracturing in a way that never happens normally is more indicative of a natural failure than tampering to you?

Really? Something happening that never happens seems natural to you?

Really?

Honestly?

Think about that for a second.

Every other AR pattern overpreasure failure ever blows the left side of the reciever off... this one, that just happens to be filmed by HK, during an HK set up test for HK promotions just happens to blow on the other side... and that's natural to you?

There are hundreds of reports out there about AR pattern rifle failures. If you talk to the right people you can even get copies of them to use as training aids in why it's not a good idea to drop your muzzle into the sand.

They have a certain pattern in them...

amusing. Applying some common sense. If you expect a receiver to fracture in such a way with failure to fracture in that way indicating foul play then obviously people who know what they are doing and want the receiver to fracture will make it fracture according to your expectations so as to avoid accusations of foul play.... I thought I made that clear earlier. There's no doubt HK knows more than most people how DI ARs crumble and until someone can categorically replicate the test in the same way and not see the same behaviour all we have against the test is the ever present speculation.

 

[Grantman]

So what I am hearing from Grantman is that only HK has experts and nobody else knows what they are talking about. Sounds about par for the course for your posts.

I didn't say that at all but I'm kinda getting used to the one eyed vibe around here. So what you are saying is the colossal company that is HK got to where they are by hiring clueless people?

 

[MCMXI]

There are hundreds of reports out there about AR pattern rifle failures. If you talk to the right people you can even get copies of them to use as training aids in why it's not a good idea to drop your muzzle into the sand.

They have a certain pattern in them...

But I'm not convinced that this is a barrel obstruction failure so perhaps a different type of receiver separation is to be expected.

Sounds about par for the course for your posts.

Why are you making this personal? If you aren't capable of having an intelligent, constructive discussion about this then flex your internet muscles elsewhere.

Unlike the majority of the anti gas piston members here, I use and like DGI and gas piston ARs. How does the expression go ... I have no horse in this race or is it I have both horses in this race? I'm only interested in the FACTS, nothing more. So far, I've read nothing substantive with regards to the HK test. From an engineering perspective, assuming no barrel obstruction, is it possible that water in the gas tube acts like a steel rod on the gas key transmitting a lot of force to the receiver? You might suggest that the piston in a gas piston AR is a steel rod and also acts on the carrier, but it has a lot of mass (compared to the water in the gas tube) that needs to be accelerated. Water being incompressible would almost instantaneously transfer close to 100% of the pressure at the gas port to the carrier (with some assumptions of course). Without looking at the receiver, carrier, bolt, gas key, gas tube etc in detail, it'd be tough to provide a diligent failure analysis.

 

[MCMXI]

I'm sure that's also why HKs "experts" most certainly know so much more about AR pattern rifles... especially when compared to lowly non-Teutonic schmuks who only carry the things around every day...

That's akin to suggesting that people that drive their cars every day and tens of thousands of miles per year, know all there is to know about them because they own and use them. Who knows more about a car, an automotive engineer who rides a bicycle to work or any of our wives, girlfriends, sisters etc. (yeah, sexist I know ... but it's true).

 

[kwelz]

1858, it isn't personal but he is leaning dangerously close to Troll territory. He has yet to back up his arguments with provable, non biased facts. In addition he has been unwilling to listen any counter arguments or facts put before him. He has also admitted to no actual experience with he rifles he claims suck.

 

[Maverick223]

That would be the same amount of pressuer that vents through the gas tube into the reciever and doesn't make it explode?

You can bend a gas tube quite easily with your bare hands into confusing shapes... not so much a reciever.

P=M*V (or Mass X Velocity = Momentum)...water has a great deal more mass. This is why there is a "thrust block" at the elbow of high pressure vessels (such as a fire hydrant pipe that bends upward from a horizontal main), if such vessel carried a lighter fluid then said structure would be unnecessary due to the lesser momentum. This is why water can perform more work than air at an equal pressure and also why more mechanical force is necessary to propel the more massive fluid. Furthermore, you didn't listen...I said that the smaller tube makes a stronger pressure vessel...it is weaker in flexure...am I missing something?

I'm not commenting on the rest, but be assured that the gas tube can contain *far* greater pressure than the upper receiver if you want to compare PSI for PSI.

The pressure in the thick steel carrier that operates the action (around 1000-1500psi according to documentation I have seen) is going to be well under that which is present in the gas tube (~15k psi at the port, depending on gas system) as the gas pulse expands there. The upper receiver is going to be far weaker than the carrier, as it is made of weaker, thinner material.

Thanks ny32182, are you an engineer or just applying some common sense with regards to pressure vessels?

From an engineering perspective, assuming no barrel obstruction, is it possible that water in the gas tube acts like a steel rod on the gas key transmitting a lot of force to the receiver? You might suggest that the piston in a gas piston AR is a steel rod and also acts on the carrier, but it has a lot of mass (compared to the water in the gas tube) that needs to be accelerated. Water being incompressible would almost instantaneously transfer close to 100% of the pressure at the gas port to the carrier (with some assumptions of course). Without looking at the receiver, carrier, bolt, gas key, gas tube etc in detail, it'd be tough to provide a diligent failure analysis.

Oh my gawd...another smart comment (not meant sarcastically, though it seems that way)...and everyone (elsewhere on the net) always thought that we were all just a bunch of retarded rednecks. Someone else understands fluid dynamics...I am not alone.

Wanted to add that it likely also has to do with the piston system draining quicker (less volume than the DGI tube) resulting in no KB...but the mass would play a factor as well.

 

[Z-Michigan]

A couple comments. Let me start by saying I'm not an engineer and I'm genuinely interested in what the engineers here have to say.

As a matter of standard AR operation, we talk about the gas port pressure being X (I think it's closer to 30k psi in a M4 type, by the way) but I assume that pressure decreases dramatically as soon as it gets into the gas tube, as it's going across a small port into a much larger volume. Is this correct?

In the discussion above, if the gas tube is full of water, why wouldn't the tube rupture where it meets the gas block, instead of propelling the water at high speed into the bolt carrier key? I guess this gets back to my question above and how much pressure the gas tube can actually take.

Second, if the plugged gas tube does shoot a jet of incompressible water at the carrier key, wouldn't that be more likely to damage the key and propel the bolt carrier backward at excessive velocity? The video looks to me more like a case or bolt failure, which would be consistent with a plugged barrel.

Finally and having no bearing on the physics, I can't watch the HK video without laughing, between the ridiculous frogman setup and the fact that he's wearing a serious face shield to test the Colt but very little to test the HK. That to me makes it look a lot like a Dateline type setup.

 

[Maverick223]

A couple comments. Let me start by saying I'm not an engineer and I'm genuinely interested in what the engineers here have to say.

Nothing wrong with that; the basic principles of the physics that are in play in the incident described above are not terribly difficult to understand, though the details of said incident are very complex and would require a thorough investigation to determine the cause. Furthermore, and I am not saying that my theory is what actually occurred...just one of many possibilities...some of the other theories are not possible or very improbable, and people keep defending them whilst practically calling others idiots in the process...which is becoming a bit annoying, at least to myself.

As a matter of standard AR operation, we talk about the gas port pressure being X (I think it's closer to 30k psi in a M4 type, by the way) but I assume that pressure decreases dramatically as soon as it gets into the gas tube, as it's going across a small port into a much larger volume. Is this correct?

Not sure the exact number, but it is very high. Every turn, orifice, constriction, and fitting will decrease the pressure, as will the loss due to the frictional coefficient of the gas tube and other components. This loss of pressure should be very small due to the short distance that the gas has to travel, but make no mistake, it will decrease in route.

In the discussion above, if the gas tube is full of water, why wouldn't the tube rupture where it meets the gas block, instead of propelling the water at high speed into the bolt carrier key? I guess this gets back to my question above and how much pressure the gas tube can actually take.

The gas block is a pretty stout chunk of steel (technical terminology used to describe a large strong member ) and would be difficult to dislodge or destroy, so I believe that is why the failure is at the bolt (which would seemingly be weak in comparison).

Second, if the plugged gas tube does shoot a jet of incompressible water at the carrier key, wouldn't that be more likely to damage the key and propel the bolt carrier backward at excessive velocity? The video looks to me more like a case or bolt failure, which would be consistent with a plugged barrel.

The key would almost certainly be destroyed in the process...but may hang on just long enough to fracture the receiver...or even become a projectile itself.

Finally and having no bearing on the physics, I can't watch the HK video without laughing, between the ridiculous frogman setup and the fact that he's wearing a serious face shield to test the Colt but very little to test the HK. That to me makes it look a lot like a Dateline type setup.

I hope he got paid...as Kermit does look like an idiot.

 

[Maverick223]

To add a little more...without a page long post...a smaller diameter pressure vessel is stronger than an equally thick larger one due to the curvature of the inner wall (this is why there are few square pressure vessels...it is inefficient). It is like the curvature of an arch...the greater the span (for a given hight) the weaker the bridge or structure. I am sure 1858 can come up with a better explanation being a ME.

 

[Grantman]

Another write up involving accuracy testing on a POF piston driven AR by David Fortier for Shotgun News magazine. Notable points is that Fortier admitted to wrongly assuming that a gas piston system would adversely affect accuracy compared with DI. His best 5.56, 5 shot group was .386" at 100 yards with 5 rounds of Hornady 55gr TAP. The 55gr Hornady performed best averaging .62" across groups.

an additional point he noted was the obvious lack of fouling in the POF vs DI design and the ease of cleaning facilitated by the gas piston design. Most people see this as common sense by looking at where the gas vents but it's relevant information when it comes to GP design nonetheless.

My thoughts? The most significant improvement was noticed
when it was time to clean. You don’t realize just how dirty the
Stoner system is until you clean a well-used POF gas piston gun.
The bolt carrier assembly was stripped and wiped clean in a matter
of a few moments. All the crud you normally find in the bolt,
firing pin, cam and everywhere else? Gone.

http://www.pof-usa.com/Fortier72006.pdf

 

[Slamfire]

The Army conducted a Dust test of a number of rifles, including the M4.

A large number of people just howled that the dust test was unrealistic, unfair, etc. Why?

Because the M4 failed.

There is nothing new in this world.

 

[Z-Michigan]

The gas block is a pretty stout chunk of steel (technical terminology used to describe a large strong member ) and would be difficult to dislodge or destroy, so I believe that is why the failure is at the bolt (which would seemingly be weak in comparison).

Sorry, I'm not talking about a failure of the gas block, but of the gas tube (stainless steel little thing), presumably close to the gas block since that's where internal pressure would be highest.

Also, I'm familiar with hoop stress as a concept, but I still can't see the gas tube being stronger than other parts, or being strong enough to contain the full barrel pressure if the gas tube is plugged. I suppose that would be an easy thing to test experimentally though. Also, does anyone know the internal pressure in the gas tube say 0.5" away from the gas port? I'm guessing it will be a small fraction of the port pressure, but I don't know.

Finally, how would the gas tube fill with water anyway? Its far end is connect to the barrel by about a .04" hole. Even though the bcg end would be submerged, how would water get in without a way to push out the air already in the tube? Or are we supposed to believe it filled up through the .04" gas port during the fraction of a second when the barrel was submerged but the bcg end of the gas tube was not?

I just don't think this adds up. I really think the Colt was fired with a plugged barrel.

 

[Maverick223]

Finally, how would the gas tube fill with water anyway? Its far end is connect to the barrel by about a .04" hole. Even though the bcg end would be submerged, how would water get in without a way to push out the air already in the tube? Or are we supposed to believe it filled up through the .04" gas port during the fraction of a second when the barrel was submerged but the bcg end of the gas tube was not?

IIRC the action was cycled allowing to air to escape and water to enter the barrel as well as the gas tube. If I am mistaken it should take some time for the relatively small gas tube to fill.