View Full Version : Gas Port Pressure v. Barrel Length Curve?
Submariner
10-03-06, 05:49
Would someone post the chart of the curve for barrel length versus gas port pressure? I have seen it elsewhere but cannot find it.
www.ar15barrels.com/gfx/223plot.gif
Courtesy Randall Rausch. Note that this is plot taken off of QuickLoad ballistics program, with an input of 60,000 psi max pressure. He provided this for illustration, but it's pretty close to actual.
Maryland AR15 site may have what you're looking for, but I can't log in from work.
ETA: Not sure how to post the image but if you cut and paste into your browser, it will come up.
Submariner
10-03-06, 11:12
That's it! Muchas gracias, jmart.
http://www.ar15barrels.com/gfx/223plot.gif
K.L. Davis
10-03-06, 11:35
As pointed out... the graph shown is a computer model of bore pressures at those locations -- the restrictive properties of the gas port reduce these pressures and actual pressures taken with sensors actually located at the gas ports are much lower.
Submariner
10-03-06, 12:11
As pointed out... the graph shown is a computer model of bore pressures at those locations -- the restrictive properties of the gas port reduce these pressures and actual pressures taken with sensors actually located at the gas ports are much lower.
Roger. I'm trying to relate how a 14.5" M4 barrel and a Colt 16" LW barrel would fit on the general curve, if at all. I have always used a standard Colt carbine buffer with the 16" and an H buffer with the 14.5" barrel. Given the recent buffer posts, do I need to rethink us standard buffer with the 16"? No problems to date in over 10,000 rounds between two carbines in Pat's classes and work-ups.
..... the restrictive properties of the gas port reduce these pressures and actual pressures taken with sensors actually located at the gas ports are much lower.
When you state above that pressures taken at the port, where are these readings taken? Just prior to the port, or just after the port, or is it possible to place a sensor on the gas tube (that would be one micro strai gauge)?
I my simple mind I can see how port diameter and dwell regulate the mass of gas delivered back to the carrier, but I could never understand why conventional wisdom stipulates carbine BCGs/extractors see a lot more pressure than rifles. I would think by tailoring port diameter you could engineer it so that regardless of barrel length, port location and port pressure, the same volume/mass of gas would always be delivered to the BCG/extractor. And if reality shows that carbines do in fact see much higher pressures at the BCG/extarctor, why don't engineers just start drilling smaller port diameters?
K.L. Davis
10-03-06, 20:23
...why don't engineers just start drilling smaller port diameters?
The first thing I thought of when I read that was a very crusty old guy that I know... on a rather memorable tear doing some testing in the Seattle area, to wit: "Engineers should stick to drivin' effing trains, too much of this crap comes from people that have never HAD to carry a gun!"
Anyway... it was funny at the time. The pressure figures I work with are taken with a clamp on gauge at the port(s) -- The most important figure is the maximum internal bolt pressure, which is a combination of flow, pressure and time.
The big reason that *whoever* does not just use a smaller port, is that the gun won't work... the problem is not that the gas pressures in the bolt are high, they need to be high to operate the rifle, because the unlocking cycle is starting too early. High pressures are a symptom, but not the problem -- and anyone that knows me, knows how I feel about that little puzzle.
... the problem is not that the gas pressures in the bolt are high, they need to be high to operate the rifle, because the unlocking cycle is starting too early. ...
I still don't get it. Oh well ......
K.L. Davis
10-04-06, 23:51
I still don't get it. Oh well ......
Have you read this (https://www.m4carbine.net/showthread.php?t=94)
I still don't get it. Oh well ......
The closer the port to the chamber the sooner pressure builds in the bolt and carrier. The brass case needs time to break the seal it has inside the chamber (shrink) before it is yanked out by the extractor. What happens in AR carbines is the extractor physically over comes the resistance of the sealed case and rips the case out of the chamber. This stresses extractors and springs and is why extractor issues appear in AR carbines long before they appear in rifle length ARs.
Yes, I've read the earlier linked thread and I understand that things happen quicker. But the way I picture it in my mind is what drives BCG cycling is the volume, or mass of gas delivered to the carrier. That volume is a factor of two things: (a) port pressure, and (b) size of the "valve" or port.
Absurd example to help illustrate my point: We always talk about port pressure as being much higher in carbine length systems, but imagine if the port were drilled out to only .001". The fact the port pressure is high is irrelevant, you simply aren't delivering enough gas to cycle the weapon. Now imagine the same port location (so still the same port pressure), but the port is now drilled out to .100". Now the system is way overgassed, there's way too much volume being delivered to the carrier. In both cases you have the same port pressure but the effects at the BCG are completely opposite. So it's obvious to me that port diameter plays a critical role in regulating the flow.
About dwell, I can see how with a shorty, and such limited dwell, that you are constrained by what you can do. Decrease the port size much and dwell kicks in to the point where you don't have enough "barrel time" left to regulate the gas and deliver sufficient volume to the carrier. But I, like a lot of people, run a 16" barrel with a carbine-length gas system, so I've got plenty of dwell to work with. It just seems intuitively obvious to me that the industry could standardize on a smaller port size for this configuration. The way I envision it a smaller volume of gas would be delivered throughout the cycle, but the total volume delivered (given the dwell) would be enough to cycle the weapon. And I would think that with the smaller volume delivered in the beginning, you would avoid the situaton where extraction is initiating while the case is still tightly gripping the chamber's walls. I would think, given the amount of dwell, you have some time to play with which would allow the case to relax it's grip. Extraction initiation would be delayed but you would still have sufficient volume needed to cycle the weapon.
We attack this issue through other means -- we either add weight to the buffer or we add weight to the carrier. The added mass accomplishes the same thing, it resists unlocking for a fraction of a second allowing the case to relax. But as many point out, that's treating a symptom, not the disease itself.
The only other thing I can point to is the Maryland Ar15 site and over there they have a table which lists min and max port diameters for various barrel lengths (middies excluded). I find it a bit strange that 14.5" and 16" barrels have the exact same diameter specs. But given the 16" barrel has 1.5" of additional dwell, it just seems counterintuitive that they would use the same port diameters. What I don't know was if the industry baselined specs for 16" barrels first and then applied the same to 14.5" barrels (which would then be undergassed, correct?), or the other way around at which point 16" barrels would be overgassed.
Final question -- does anyone know the industry standard for mid-length port diameters?
K.L. Davis
10-08-06, 21:41
...snip...
The port pressure, size of the port, dwell time and barrel dia all conspire to one thing: the peak pressure inside of the bolt assy.
The carbine pressure is about 50% higher than the rifle pressure, but this is a symtom of the problem -- as the lock time is much shorter, the gun is trying to unlock sooner, the chamber pressures are higher and it requires more pressure to operate the rifle -- this is exacerbated by the fact that once the bolt is unlocked, the higher pressures accelerate the bolt faster.
It is like this... if a car requires 115 BHP to go 60 MPH, it will reguire more BHP if the rolling resistance is increased... up hill, heavier, drag, whatever.
The early operation of the short gas system makes the rifle operate with more "resistance" -- so more pressure (horsepower) is needed to make it run.
Submariner
10-09-06, 05:42
We attack this issue through other means -- we either add weight to the buffer or we add weight to the carrier. The added mass accomplishes the same thing, it resists unlocking for a fraction of a second allowing the case to relax....
... What I don't know was if the industry baselined specs for 16" barrels first and then applied the same to 14.5" barrels (which would then be undergassed, correct?), or the other way around at which point 16" barrels would be overgassed.
This is what I was attempting to discern from the chart. Wrong data.
Colt originally installed standard buffers with 16" carbine barrels. M4's came with standard buffers, briefly, then H buffers to slow unlocking. Now Colt sends 16" carbines (6520) with H buffers. Is that because it is chaper for them to buy just one buffer for all carbines or because the 16" barrel needs that additional resistance? If the latter, is an H buffer enough or should it have an H2 due to the additional dwell time? One reliable source, Colt Armorer Instructor Dean Caputo, advises me the standard buffer is enough. In fact, most of his 50 Colt Commandos run with standard buffers.
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