So gas port can play a factor as well is what I get from the above.
Do you have anything to quantify this (not questioning just curious). Like 10.3 at .070” versus 11.5 at .075” or whatever.
Is it fair to say that it would cause a 10%, 50% or no difference for example?
I just read disingenuous and wonder how we have been misled and if so by how much?
What do you think?
God Bless,
Brandon
I'd hesitate to speculate on the "by how much" part. I also think there's a difference in theory and practice. I work with engineers all day and sometimes there's a tendency to let perfect become the enemy of good enough. There's also a tendency to make assumptions sometimes because you can't get the data. What assumptions are made and when they are made can be very different.Originally Posted by BWT
My point is that the motive force that acts on the BCG in an AR15 is gas, both pressure and volume and that they are inversely proportional to one another depending upon the port size. Think of it this way: when we put an adjustable gas block in a gun to "tune" it, what are we changing? It's the flow of gas into the gas tube and ultimately to the BCG. Therefore, this must play an important part in the reliability and function of the system.
The disengenuous bit comes in when we say something like "An 11.5" barrel has a 40% greater dwell time than a 10.3" barrel." Oh really? Well, first we have to define what is meant by dwell time, but rather than arguing the symantics of that, lets look at it in simpler terms, observable factors such as bolt speed, recoil impulse, etc. If the 10.3" barrel has a DD sized 0.083" gas port, then yeah it has less dwell time than an 11.5 with a .065" gas port. Hell, it has less dwell time than a 10.3 with a 0.070 gas port.
So, when someone makes that statement, it's a bit sensationalized unless we know what port sizes were used in the comparison.
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Semper Paratus certified AR15 armorer
I didn’t bring up Boyle’s Law because I haven’t confirmed if the gases are supersonic or not. I forget the actual number, but I think the expansion rate of smokeless powder exceeds 5500 fps. But I don’t know if that velocity is supersonic at the temps produced by the burnt powder. If it’s subsonic, Boyle’s Law applies. If it’s supersonic, things change. When a supersonic fluid passes through a restriction, velocity drops to the speed of sound.
Gas port diameter is extremely important to proper function. As I pointed out earlier, the gas port controls flow according to pressure and port diameter. (Strictly speaking, port length also plays a role, but in the AR, a minor one because barrel walls are relatively thin.) When a shorter gas system is used, the gas enters the gas port at higher pressures. If the same gas port diameter was used in a carbine gas system as a rifle gas system, flow rate would drastically increase. The “fill rate” would be too quick. Too much gas would enter the expansion chamber too quickly and the resulting carrier speed would be excessive.
So, a shorter gas system needs a smaller gas port to keep the fill rate within allowable limits. When a suppressor is added, gas flow out the muzzle is restricted and the pressure in the bore takes longer to drop. This increases pressure at the gas port which increases flow which decreases full time resulting in increased carrier speeds. Increase (or decrease) carrier speed enough, it has a negative impact on timing and thus, function. The system is then out of balance.
I’ve experimented with 10.5” and 11.5” barrels using an adjustable gas block, suppressed and unsuppressed. The system was closed until the action would eject but not lock back on an empty magazine. Then, opened until the action would eject and lock back. The difference was one click (which is one third rotation of the adjuster screw).
Unsuppressed, the gas block needed to be opened more compared to suppressed. I believe the difference was one or two clicks.
More interesting to me was testing with different buffer weights. The same gas setting was needed for full function (eject and lock back on empty mag) whether a carbine buffer was used, H, H2 or an A5H2. The difference was in felt recoil. Felt recoil with the carbine buffer was sharp and uncomfortable, especially when shooting in just a tee shirt. But as buffer weight increased, the softer felt recoil became. Softest of all, was the A5H2 which weighs about the same as a rifle buffer. I’d like to note this was true whether the AR was tested with a suppressor or without.
Because of this testing, I’ve come to the conclusion that a carbine buffer is simply too light when full power NATO spec ammo is used and the H2 buffer is a better choice than the H. Better still is the A5H2 and rifle buffers.
A carbine buffer might be ok when using low pressure ammo. Low pressure ammo reduces gas drive. But lowering the gas drive could throw the system out of balance because the reciprocating mass might now be too much. However, changing reciprocating mass changes the momentum of the BCG which can impact feeding and timing.
Keep in mind, small changes can keep things within the normal span of operation, but with a smaller margin at one end or the other. With small changes, imbalances in the system may not be enough to make themselves known.
(I learned a lot from Will Larson. His depth of knowledge about ARs and other families of weapons was incredible. We are diminished by his death.)
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I do not have the depth of knowledge of the others here, but it seems to me this discussion parallels many of the mid-length pro/con discussions. And the way I understand the sequence of events the 7" gas system was built to accommodate the 10" (subsequently 11.5") barrel but then was carried over to many more numerous examples of 14.5" and 16" barrels. And many of those probably have larger gas ports than optimal, to ensure things work with all varieties of ammo.
I had my first mid-length barrel about 20 years ago when 16” carbine hbars and fixed stocks were high speed.
I’ve had a lot of barrels, experimented with a lot of different things, and have built a LOT of ARs for a lot of people from competition to swat use.
Some observations:
In the interest of reliability, the ammunition should be the starting point. Not the afterthought.
A properly dimensioned and smooth chamber is much more a factor of reliability than gas port location.
Gas port size is much more important than gas port location.
Buffer weight needs to be sufficient enough to ensure momentum to fully cycle to the rear. (This is the part nearly nobody understands and goes hand in hand with ammo comment above)
Once the gas port passes the peak of pressure, moving it closer and closer to the muzzle only makes it less tolerant of cold temperatures.
The three most groundbreaking and effective reliability improvements to the carbine setup are the H2 buffer, the Colt copper extractor spring and black insert, and the Gen M3 Pmag. All the other bs is marketing fluff.
OK, right off, I'll confess I don't know spit about how ARs work. But I read through this thread, mainly out of curiosity. And given all the discussion of pressure, and distances (placement) of the gas thingy from one end of the barrel or the other, my curiosity turned into puzzlement. Does the powder volume and burn speed not have a significant impact on this pressure wave (duration and where the peak is relative to the holes in the barrel)? Enough to "matter" compared to distances and diameters and such? Powder choice can certainly result in "differences" in handguns re: barrel length.
This has been a very frustrating thread to read. It's been more confusing than helpful, making what's really going on more difficult to understand than it's been enlightening. Not what I usually get from P-F.
To answer your question, yes, powder burn rate affects the pressure profile as the bullet travels down the barrel, and can affect the operation of the gas system significantly. You see this when some slower burning powders are used in older military-based designs. Hornady even included a warning when it launched the Superformance line about using the ammo, which has powders specially engineered to maintain higher pressure for a longer duration ("area under the curve") with each cartridge, about using it in gas guns. You see it going the other way when people load fast, pistol-speed powders in .300 BLK. That led to the evolution of the "pistol" length gas system, to capture the pressure that's available from these quick burning powders in short AR barrels.
As far as the more general question of burn speed vs. barrel length (ETA: I'm thinking about rifles for the rest of this paragraph), my understanding is that whatever powder and load provides the best velocity from a given cartridge will provide the best velocity (compared to other powders and loads) at any barrel length. So you don't load a fast powder for a short barrel and a slow powder for a long barrel. You load the powder that gives the best combination of accuracy and velocity. Its velocity performance relative to other loads will likely be maintained at other barrel lengths. Accuracy may be a different result, though, because that depends on the timing of barrel harmonics relative to when the bullet leaves the barrel, and that will change with barrel length. So a powder that more consistently delivers the optimum relationship of bullet acceleration to barrel harmonics may be different (but not radically different) at significantly different barrel lengths. Or it may not. Every barrel is different if you can measure precisely enough.
Last edited by OlongJohnson; 06-03-2022 at 05:23 PM.
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Not another dime.
Thanks very much!
Dwell down, blow time, gas explosions, I'm not smart enough to argue one way or the other but I have to agree with @ASH556 about what a difference a properly sized gas port will make. I have a lot of time and rounds through my Colt 6933 and consider it to be the short upper that all others are judged by, by my Crane spec 10.3s leave nothing on the table in regards to reliability.
Also, Orion Training Group posted this on IG this morning and I laughed so hard I choked on my coffee. It's just a meme so no need for hurt feelings.
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