Does the Gas Port Control Pressure?

[bfoosh006]

I might have some answers. The best solution/answer would be to instrument an AR and measure chamber pressure, barrel pressure at the gas port and then pressure in the bolt carrier while firing the rifle. Then vary the parameter(s) you guys are arguing about and get some more measurements. Not a trivial experiment to setup but thankfully you all paid your taxes and Uncle same did that experiment for you. The website dtic.mil is a wonderful resource for nearly all technical data from the military research branches that Uncle Sam has deemed acceptable for the proles. Back in 1971 the Internal Ballistics Lab at Aberdeen did such and experiment on and M16A1.

http://www.dtic.mil/dtic/tr/fulltext/u2/731218.pdf

Now this paper gets right down into the weeds as far as the thermal and gas dynamics goes. Its gritty, I spent a week about two years ago grinding through this paper and several other the author, M.W. Werner and his colleagues trying to creating my own modern version of his simulation in Python (using Sci-Py and Num-Py) but I think most of the questions here in this thread can be answered in a few graphs of the real world data taken as part of the research they did to create a computer model of the M16 gas system.

Page 20 has a nice graph showing the pressure vs time graph of the chamber, gas port, and bolt carrier pressure all overlaid on the same graphe for the standard M16A1

Now how does port diameter effect that carrier pressure?

Page 29 lower graph has the pressure measured in the bolt carrier at 5 different gas port diameters.

There is a whole bunch more data and modeling that goes on in that paper. They look at a few other variable but those two graphs I think answer most of the questions in this thread. Feel free to read the whole paper its pretty good. I wish the images had scanned better but the report is type written from 1971 and the image are no doubt photo's glued into the report.

Heck of a good read, thank you for sharing and I have saved this one on the computer.

 

[Varminterror]

Flow would reverse and push gas back through the port to be vented through the muzzle.

That’s not how pressure works - pressure exerts a uniform force in all directions. The mass only goes where the path of least resistance (aka path of lowest pressure drop) lets it go.

Such the flow would never reverse. The statement copied here is a fundamental flaw in understanding of pressure differential vs. mass flow. As the pressure drops, it drops uniformly in all directions. In the dynamic case, the pressure only exists while the combustion then expansion is driving energy to the system. As that mechanical energy is purged to atmosphere, the pressure just drops in the system.

And of course, we haven’t touched at all on the compressibility factor for any aspect of the system - we’re only considering the pressurization of the bore and gas system by the expanding gases behind the bullet - wholly neglecting the dynamic response of the air mass in the bore which has to get out of the way of the bullet as it leaves.

The system really is a lot more interesting than it might seem on the surface - but also a lot more misunderstood as well.

 

[Varminterror]

Mechanical devices are programmed. The diameter of the gas port is chosen because flows a certain amount of fluid over a certain length of time under a certain level of pressure. Gas port diameter is an important part of the AR's programming. So is spring rate of the action spring and the mass of the carrier & buffer

I wouldn’t say in “programming,” these are aspects of the “design.” The term programming likely elicits an inference of a intelligent dynamic response to a variable stimulus. I wouldn’t say a sledge hammer was “programmed” to deliver more force than a framing hammer, but it certainly is designed to do so.

The wording regarding pressure and flow here is very close - the orifice size represents a certain resistance to flow for a given substrate (design criteria for fluid density, viscosity, and compressibility) at a given expected operating pressure (another design parameter). Increase the flow - velocity - by double, and you increase that resistance by quadruple, so the greater the velocity - which is held by simple bernoulli principles - the MUCH greater the resistance to velocity, aka pressure drop. So if I cut the flow area in half, increase my velocity by double, I increase my pressure loss across the orifice by 4. In this way, orifice restrictions are often used for surge protection/inhibition. It’s not as effective as a dynamic mechanical or automated regulator, but it does encourage stabilization.

 

[MistWolf]

I wouldn’t say in “programming,” these are aspects of the “design.” The term programming likely elicits an inference of a intelligent dynamic response to a variable stimulus. I wouldn’t say a sledge hammer was “programmed” to deliver more force than a framing hammer, but it certainly is designed to do so.

Tools are designed, but they are programmed as well. Take your framing hammer vs sledgehammer. Weight, leverage, striking surface are all part of the programming to ensure type of hammer will perform it's function efficiently. Think of it as firmware. Better examples of mechanical programming are such devices as carburetors, cam shafts, differentials and transmissions. This isn't to imply that saying tools are designed is wrong or even less correct. Just like it would not be wrong to say computer programs are designed.

...bernoulli principles...

What's interesting, Bernoulli's Principle only applies to flows at subsonic speeds. However, the gas flow from smokeless powder is supersonic. (Of course, velocity of the flow in the bore is limited until the bullet it's pushing gets out of the way.) When the gas flow is supersonic, the rules change. When supersonic air flows through the gas port, its velocity decreases and pressure and density both increase.

 

[WrongHanded]

...a spitting contest between myself and Varminterror.

And this is where we learn.

Fascinating subject guys. I'll never look at an AR the same way again.

 

[someguy2800]

Tools are designed, but they are programmed as well. Take your framing hammer vs sledgehammer. Weight, leverage, striking surface are all part of the programming to ensure type of hammer will perform it's function efficiently. Think of it as firmware. Better examples of mechanical programming are such devices as carburetors, cam shafts, differentials and transmissions. This isn't to imply that saying tools are designed is wrong or even less correct. Just like it would not be wrong to say computer programs are designed.


What's interesting, Bernoulli's Principle only applies to flows at subsonic speeds. However, the gas flow from smokeless powder is supersonic. (Of course, velocity of the flow in the bore is limited until the bullet it's pushing gets out of the way.) When the gas flow is supersonic, the rules change. When supersonic air flows through the gas port, its velocity decreases and pressure and density both increase.

It’s impossible to have a conversation with you beacause the way you think physics works is so strange it defys comprehension. The answer to your questions is in the excellent report on the last page, and the answer is yes port diameter affects pressure in the carrier quite dramatically. When you look at those pressure traces keep in mind 0 is the point where the bullet left the muzzle.

 

[MachIVshooter]

Mechanical devices are programmed.

Calibrated is more appropriate verbiage.

At any rate, there seems to be a lot of confusion about two simple mechanical devices that are used to control aerodynamic/hydrodynamic flow.

The one which is the subject of this thread, the orifice, has been more than sufficiently explained by Varminterror. I can't spell it out any better.

The other type, which some people seem to have conflated with an adjustable orifice, is the pressure regulator, which uses a calibrated, often adjustable valving system to stop flow from the high/input side when a certain pressure is reached on the low/output side. A pressure regulator will prevent equilibrium between two parts of a closed system, irrespective of time. Also, unlike an orifice, which is designed specifically to restrict flow, a regulator is designed to maintain a constant pressure and flow at the output.

 

[mcb]

Tools are designed, but they are programmed as well. Take your framing hammer vs sledgehammer. Weight, leverage, striking surface are all part of the programming to ensure type of hammer will perform it's function efficiently. Think of it as firmware. Better examples of mechanical programming are such devices as carburetors, cam shafts, differentials and transmissions. This isn't to imply that saying tools are designed is wrong or even less correct. Just like it would not be wrong to say computer programs are designed.

I am a mechanical engineer by schooling and a design engineer by profession and I have never programmed one of my mechanical designs, nor heard a colleague refer to programming a mechanical design. I have certainly done a lot of programming over the years writing code to help design parts like cams, or code dynamics simulations of more complex mechanisms to help in the design of the system. I have also done a far amount of programming of computers or micro controllers to control an electro-mechanical systems but the parts themselves are designed. The function of a purely mechanical system is designed not programmed. There are very few purely mechanical systems that reach the level of complexity to be considered programmed or programmable.

What's interesting, Bernoulli's Principle only applies to flows at subsonic speeds. However, the gas flow from smokeless powder is supersonic. (Of course, velocity of the flow in the bore is limited until the bullet it's pushing gets out of the way.) When the gas flow is supersonic, the rules change. When supersonic air flows through the gas port, its velocity decreases and pressure and density both increase.

Actually gas flow in a gun barrel is entirely sub-sonic. Remember speed of sound in a gas is dependent on temperature and the propellant gases in a gun tube are at high temperature raising the speed of sound in the gas considerably. When you start looking at what limits how fast a gunpowder propelled projectile can be fired the theoretical limit basically comes from the fact that you cannot push a bullet faster than the shock wave propagates through the propellant gases. The theoretical limit with most conventional propellants is about 6000 fps though the practical limit for most small arms is closer to 5000 fps. And you can go faster with special propellants.

Now the gas system is another issue. When the bullet passes the gas port the high pressure/temperature propellant gases rush into a gas system that is currently at only ~14.7 psi (absolute) at that interface between high and low pressure/temperature gases you can create a shock wave that will travel down the lower pressure portion of the gas tube. I forget if it was Werner or someone else at Aberdeen or Picatinny that did some research on the effects of shock waves in the gas system and the flow problems they could create. If I can find that paper again I will link it in.

This shock wave problem is also the reason air rifles cannot get much above super-sonic projectile velocities. Some spring air-guns do achieve velocities slight above the speed of sound in ambient air only due to the fact that they heat the high pressure air in the barrel through adiabatic compression. It is also the reason light-gas-guns exists. If you push the bullet with a less dense gas (shock-wave velocity is inversely proportional to molecular mass) like helium or hydrogen you can raise the velocity of the shock wave in the high pressure gas and thus fire projectiles at much higher velocities then is achievable with the relatively heavy molecules formed by burnt gun powder. NASA and the military have light gas guns operating at over 22,000 fps muzzle velocity.

 

[MistWolf]

Calibrated is more appropriate verbiage.

Calibrated works. It's what more folks are familiar with.

I am a mechanical engineer by schooling and a design engineer by profession and I have never programmed one of my mechanical designs, nor heard a colleague refer to programming a mechanical design.

I had an autoshop teacher in high school that was a retired Ford engineer. He (along with the text books we used) talked about how venturis, diaphragms, flow paths etc. were used to mechanically program carburators. Over the years, I've had other text journals speak of the same thing. Of course, back in my high school days, computers still ran on reel to reel tapes and IBM cards. Digital computers in automobiles was still a pipe dream.

Fun Fact: The first computers were human. The word "Computer" was first used to describe one whose "occupation was to make arithmetical calculations" and dates back to the mid 1600s.

unlike an orifice, which is designed specifically to restrict flow, a regulator is designed to maintain a constant pressure and flow at the output.

I was trying to figure out a simple way to describe a regulator and here it is.

 

[MistWolf]

It’s impossible to have a conversation with you beacause the way you think physics works is so strange it defys comprehension. The answer to your questions is in the excellent report on the last page, and the answer is yes port diameter affects pressure in the carrier quite dramatically. When you look at those pressure traces keep in mind 0 is the point where the bullet left the muzzle.

It affects how fast the pressure is allowed to build in the carrier.

 

[MistWolf]

Actually gas flow in a gun barrel is entirely sub-sonic. Remember speed of sound in a gas is dependent on temperature and the propellant gases in a gun tube are at high temperature raising the speed of sound in the gas considerably. When you start looking at what limits how fast a gunpowder propelled projectile can be fired the theoretical limit basically comes from the fact that you cannot push a bullet faster than the shock wave propagates through the propellant gases. The theoretical limit with most conventional propellants is about 6000 fps though the practical limit for most small arms is closer to 5000 fps.

I was aware that temperature affects the speed of sound, but did not take into consideration the temperature of the gasses would be enough to raise the speed of sound to such a level. You wouldn't happen to have the data that shows this?

 

[Varminterror]

Under a minute with Google shows combustion temperature somewhere between 1900-2500k (at the muzzle), and also gained access to a speed of sound “calculator” which yields something around 3,000fps at that temp.

 

[MistWolf]

Thanks, VT. I learned something new today.

 

[MistWolf]

That’s not how pressure works - pressure exerts a uniform force in all directions. The mass only goes where the path of least resistance (aka path of lowest pressure drop) lets it go.

Right. My example is if the carrier were sealed and did not vent pressure. The flow would stop when pressure equalized, then reverse to flow back into the bore as the pressure in the bore continued to drop after the bullet uncorked the muzzle.

It would be like filling an air tank from a compressor, then disconnecting the air hose from the tank without closing the valve.

 

[Varminterror]

Me too - or at least close enough to give it credit as such. I knew the hyper-temp expanding gas had extreme shock wave velocity potential, but didn’t have a bearing on the actual value. In other words, I knew it was fast, but didn’t know how fast.

I doubt the 3,000fps I calculated is accurate to the in-bore speed, but but it’s close enough for me to be happy as confirmation of the range of wave speeds @mcb cited above.

 

[MistWolf]

I need to thank mcb as well. mcb, I learned a few things from your post. Along with opening my eyes to my not taking temperature into account, I didn't know why airguns were limited in velocity.

mcb- does that stand for Monster Control Bureau?

 

[mcb]

Thanks guys. I am far from an expert in solid propellant or internal ballistics my mechanical engineering background is primarily dynamics/solid-mechanics. Fluid and thermal is the "dark side" of mechanical engineering IMHO but I have the fundamentals from undergrad and have been learning as much of this as I can cause I do run into it more and more in my professional career. But Newton's second law is so SO much more solvable when gases are not involved.

I started with read all the background information from Quickloads it a pretty good overview and made the program more intuitive and useful. I have also been grinding through the book Ballistics: Theory and Design of Guns and Ammunition by Donald E. Carlucci & Sidney S. Jacobson and a fair number of internal ballistic papers I have found on dtic.mil and other sites on the web. I actual meet Carlucci through the coarse of my previous job.

I have a S&W 610 but I am not sure the Monster Control Bureau would have me I think I like my S&W 625 better.

 

[someguy2800]

If you look at the pressure traces in the ducument linked on the last page you’ll notice there is a significant delay between when the pressure reaches the gas port and when it reaches the carrier chamber. The reason for that is because of the speed of sound in the gas tube. A pressure wave cannot travel faster than the local speed of sound. Actually that is the definition of the speed of sound. When gas initially hits the gas tube the gas in the tube is relatively cool and low pressure so the local speed of sound is slow. As the pressure and temp climb the local speed of sound increases so the shockwave will travel faster as the tube and carrier fill. You can see from the traces that the bullet has actually left the muzzle before the pressure reaches the carrier due to the speed of sound in the gas tube.

Calibrated works. It's what more folks are familiar with.

I had an autoshop teacher in high school that was a retired Ford engineer. He (along with the text books we used) talked about how venturis, diaphragms, flow paths etc. were used to mechanically program carburators. Over the years, I've had other text journals speak of the same thing. Of course, back in my high school days, computers still ran on reel to reel tapes and IBM cards. Digital computers in automobiles was still a pipe dream.

Fun Fact: The first computers were human. The word "Computer" was first used to describe one whose "occupation was to make arithmetical calculations" and dates back to the mid 1600s.


I was trying to figure out a simple way to describe a regulator and here it is.

I apologize for being rude. We are a generation apart so programming specifically related to computers to me since I grew up in the digital age. Communication gap. My comments weren’t very high road.

 

[someguy2800]

It affects how fast the pressure is allowed to build in the carrier.

This is from the linked article. There is a about a 250 psi difference in peak pressure between a .092 and .094" gas port but the peaks are at roughly the same time. The .092 peaks about 6 thousands of a second after the bullet leaves the muzzle and the .094" is about 7 thousands of a second (7 milliseconds). The pressure rise is pretty much identical though up to 2.4 kpsi. Between all 4 port diameters tested the peak pressure is reached at roughly the same time, +/- .1 ms. So it appears to me the biggest factor in changing port size is the pressure reached in the carrier, not the timing of that pressure.

 

[MistWolf]

If you look at the pressure traces in the ducument linked on the last page you’ll notice there is a significant delay between when the pressure reaches the gas port and when it reaches the carrier chamber. The reason for that is because of the speed of sound in the gas tube. A pressure wave cannot travel faster than the local speed of sound. Actually that is the definition of the speed of sound. When gas initially hits the gas tube the gas in the tube is relatively cool and low pressure so the local speed of sound is slow. As the pressure and temp climb the local speed of sound increases so the shockwave will travel faster as the tube and carrier fill. You can see from the traces that the bullet has actually left the muzzle before the pressure reaches the carrier due to the speed of sound in the gas tube.

Yes. This is why I realized the carrier isn't getting the hot gas that some critics say it does.

I apologize for being rude. We are a generation apart so programming specifically related to computers to me since I grew up in the digital age. Communication gap. My comments weren’t very high road.

No worries. Like the rest of us, you're participating in this thread to discuss and learn.

 

[Varminterror]

Note the differences in the two pressure graphics in this thread - one reflects bore pressure as predicted by Quickload, the other the calculated and measured pressure in the gas tube. The second reflects the effect of port diameter upon gas tube pressure, but what is REALLY remarkable - and the entire purpose for this thread - is the difference in the bore pressure and the gas system pressure. Something on the approximate order of 2500psi in the gas system, as compared to no less than 20,000psi in the bore at the port position (more for shorter gas system lengths). Effectively, we’re talking about an order of magnitude difference between bore pressure and gas system pressure - I honestly would not have guessed that much. But it certainly shows the gas system doesn’t reach full bore pressure.

 

[mcb]

Yep, that is why gas port diameter (or use of an adjustable gas block) is used by nearly all manufactures to tune the AR-15/10 gas system to new configurations (cartridge, barrel length, gas system length, etc). Gas port diameter is a huge driver of the system's performance more so than any other single "dial" you can turn while tuning the system. Carrier/buffer mass probably being the next most influential "dial" you can turn.

 

[Varminterror]

^ such has been my assertion for ~20yrs. This thread, and the precursor which lead to it, have not been the first time being in this discussion online, and I honestly don’t expect it will be my last. I’m also certain I’m not the only one out there who has gone these rounds trying to explain the principles to others - and not the only one often getting ridiculed by stubborn folks who don’t work in the field. Not many folks really have an understanding of fluid dynamics, but a LOT of AR owners have the same misconception of the applicable concepts. At least in this particular iteration, the facts came to bear. I’m gonna save a link to this thread with those future iterations in mind.

 

[luzyfuerza]

The function of the gas port is actually significantly different than I think has been described above.

When a compressible fluid flows through a given orifice (like an AR gas port) higher upstream pressures create higher mass flows (assuming constant downstream pressure). UNTIL the upstream pressure reaches the CRITICAL PRESSURE. At this critical pressure, a standing shock wave forms in the throat of the orifice, and mass flow cannot increase any further (the nozzle is choked) no matter how much higher upstream pressure is raised.

Of course, pressure downstream of the gas port in an AR isn't constant during the firing cycle. However, as long as the ratio of upstream and downstream pressures stay above the CRITICAL PRESSURE RATIO, the gas port will remain choked.

On page 10 of the excellent report referenced above, the little * marks above the parameters at the gas port indicate operation at critical pressures.

Bigger orifices, when choked, will pass more mass.

Mass flows through the gas port of an AR are steady as long as the gas port is operating above the critical pressure ratio, whatever that ratio is for a particular fluid temperature.

This fact creates a constant mass flow into the gas system for much of the firing cycle; a flow rate that does NOT vary with barrel gas pressure.

 

[luzyfuerza]

BTW, the gas port is the only choked nozzle in the system. Has anyone ever heard of opening up the inner diameter of your gas tube to let more gas into your system? That's why gas port size IS the most important factor in gas system design.

I haven't run any calculations to confirm this, but I'd be VERY surprised if there is adequate pressure differential to create a choked nozzle anywhere downstream of the gas port. As noted above, the gas block, gas tube, and expansion chamber don't have working pressures anywhere close to barrel gas pressure. As a result, standard Bernoulli equations can be used to model the behavior of gases inside these components.