View Full Version : Calibrated overgassing
StainlessSlide
06-06-14, 11:12
I've been studying the gas and recoil systems of our favorite machine for a while now, and hope to post some quantitative results later in the year. For now, let me float my idea for increasing reliability by means of a controlled or calibrated overgassing.
We know that some overgassing is necessary for the carbine to function under stress. If the gas pressure profile in the bcg cavity is barely sufficient for lockback under perfect circumstances, then fouling, external debris and heat will result in a short stroke.
On the other hand, we all know the drawbacks of too much gas pressure in the bcg.
My idea for finding a calibrated happy medium is to use an adjustable gas block to tune the system to an extra-power recoil spring, and then use a standard weight one. For example, I might use the adjustable block to regulate the minimum gas for the Sprinco red spring or the Wolff xp spring, and then change them for a standard weight spring for actual use.
Some users do this by opening the adjustable gas block a certain number of turns (or fractions thereof) after they have achieved minimum function with the spring they intend to use, but the amount of extra gas flow that results depends on a lot of thing like the size of the gas port, the amount of restriction that the block has already, etc. The Xp recoil spring method provides a measured overgassing.
I'm always interested in criticism of my ideas from thoughtful and experienced sources.
I like the basic idea...
But aftermarket springs disgust me. I suppose it's an option for guys who aren't really good at feeling how the gun is running... or don't have a broad reference of ARs they've shot to tell where a gun is running.
What kind of quantitative results are you looking to gather? It would be slick if there were some instrumentation you could hook up to the gas tube and get a reading of gas volume and pressure over time. Historically it's been measured in cyclic rate on full auto.
StainlessSlide
06-06-14, 11:51
There's a paper here which includes measurements of the bcg pressure profile
http://www.dtic.mil/dtic/tr/fulltext/u2/731218.pdf
But it's about the rifle length gas system. I don't know about his theoretical model, but the graphs are interesting. Note that the bcg cavity pressure does not fall rapidly when the bullet exits.
It would be nice to know that with gas system tuned to a 30% stronger spring (like the MGI spring), you could spend a windy week in the desert with a sugar-cookie-looking carbine and still get function. Of course in a complicated system like this there are a million variables like chamber/throat size, ammo, size of debris, and many more. But it would be a start at least, and comparisons could be made across barrel lengths and recoil systems (carbine vs A5, for example). That couldn't be done by counting clicks on an adjustable block.
Cool link. I'm going to have to take that in over the weekend.
StainlessSlide
06-06-14, 13:58
I haven't looked at that paper for a while, but the graphs show something surprising.
They set t=0 at the time when the pressure wave first hits the bcg cavity (about .25 milliseconds after the bullet passes the port). This means that the bullet exits the muzzle at about t=.24 ms. At this point the bcg cavity pressure is still RISING. It looks like Bcg cavity pressure increases by 20-30% AFTER the bullet exits.
This seems to be because the pressure in the barrel (at the port) is still higher than the bcg cavity pressure for some time after bullet exit.
It looks like Bcg cavity pressure increases by 20-30% AFTER the bullet exits.
I would suggest that this is an expansion chamber effect from the gas tube. The port in the barrel is much smaller than the inner diameter of the gas tube, so the tube itself is acting like an elongated expansion chamber. Very interesting in either case.
I haven't looked at that paper for a while, but the graphs show something surprising.
They set t=0 at the time when the pressure wave first hits the bcg cavity (about .25 milliseconds after the bullet passes the port). This means that the bullet exits the muzzle at about t=.24 ms. At this point the bcg cavity pressure is still RISING. It looks like Bcg cavity pressure increases by 20-30% AFTER the bullet exits.
This seems to be because the pressure in the barrel (at the port) is still higher than the bcg cavity pressure for some time after bullet exit.
Not surprising at all. Physics tells us that the gas pressure doesn't reach the expansion chamber until after the bullet exits the muzzle. Part because the gas port & tube restrict flow, part because gas velocity is slowed to subsonic speeds as it passes through the port.
The pressure at the port is higher than in the expansion chamber because it takes very little pressure to get the carrier moving which in turn, keep the pressure low.
Thanks for the link! It's what I've been looking for in my research
BufordTJustice
06-06-14, 19:40
I actualy do the same thing, but with buffer weight. I calibrate the gas block to run with a heavier buffer and then reduce the weight of the buffer to give me margin.
I found this on page 25 which jumped out at me:
"The port diameters used in the test were 0.030, 0.060, 0.092, 0.094, and 0,120 inch. The effect on the functioning of the weapon indicates that the practical limits for the port diameter lie between 0.060 and 0.120 inch, assuming no other parameter is changed. With the smaller port the bolt-carrier buffer failed to reach the rear of the receiver and with the larger port the buffer button was compressed to the extent the carrier key impacted the rear of the receiver."
I wonder how many 16" guns with carbine gas systems (that are over-gassed) actually have the carrier key contacting the rear of the receiver?
And this from page 32:
"The vents relieve the pressure in the bolt carrier cavity so that it does not apply force on the cam pin after unlocking and may retard fouling of the gas system."
StainlessSlide
06-06-14, 20:11
I'm glad you found the link useful. The graph in figure 8 has the y axis labeled incorrectly- the pressures are too large by a factor of 10. You can see this by comparing with figure 5 and the Summary data table.
On examining fig. 8, I can see that I made a misinterpretation above. The bullet passes the gas port about about t=-.25 ms, and the pressure shock hits the gas port at t=0. Since the gas tube is about a foot long, this gives a shock velocity of about Mach 4. This makes sense, because strong shocks are always supersonic. Of course the shock velocity is different from the velocity of the gas behind the shock.
Anyway, my misinterpretation was in starting my estimated dwell time (between port and muzzle) from t=0 rather than from t=-.25 ms. With this correction, the pressure hits the bcg at about the time of bullet exit, which is at about t=0.
As you say, bcg pressure will be lower than port pressure in this interval because of the port restriction and the quick increase in cavity volume. What is surprising is that the port pressure drops so slowly after the bullet exits, leading to continued flow into the bcg for a whole millisecond after bullet exit.
Great idea about buffer weight.
StainlessSlide
06-06-14, 20:37
And this from page 32:
"The vents relieve the pressure in the bolt carrier cavity so that it does not apply force on the cam pin after unlocking and may retard fouling of the gas system."
Another surprising thing is that the vents don't do more. Fig. 2 shows that venting begins at about t=1.8 ms, but looking at Fig. 8, there is not an immediate effect on cavity pressure rate of decline.
Markm has a good point that the gas tube is part of the expansion chamber also. A 13" gas tube with a id of .125 will have an interior volume of about .16 cu in., while I would estimate the bcg cavity proper to have a volume of about .04 cu in.
BufordTJustice
06-06-14, 20:43
Another surprising thing is that the vents don't do more. Fig. 2 shows that venting begins at about t=1.8 ms, but looking at Fig. 8, there is not an immediate effect on cavity pressure rate of decline.
Markm has a good point that the gas tube is part of the expansion chamber also. A 13" gas tube with a id of .125 will have a volume of about .16 cu in., while I would estimate the bcg cavity proper to have a volume of about .04 cu in.
Correct. The paper even states that the vents only act on the back half of the pressure curve, with essentially zero effect on the front (initial) portion of the curve.
This shows that the LMT enhanced carrier can only partially address an over gassed gun and that it also must be addressed at the front with an adjustable gas block.
Also, I would imagine that the internal geometry of the gas block and the restriction/adjustment mechanism also seems it would bear directly on the function of the gas system. The paper stated that the initial volume is a critical figure for function.
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Given a properly manufactured barrel and quality ammunition, this thread is as useful as a fart in a bottle.
Considering that "fart in a bottle" is what makes the AR run, it's pretty damn useful
BufordTJustice
06-07-14, 05:39
Considering that "fart in a bottle" is what makes the AR run, it's pretty damn useful
Agreed. This is getting back to basics. Understanding these concepts will aid with everything from troubleshooting to suppressor optimization.
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Then by all means open it up and take a wiff. :p I bought a BCM Socom upper and since it is my first carbine gas gun or this sort, do you think it would run ok with an H3 and sprinco blue that I have laying around?
BufordTJustice
06-10-14, 20:13
Then by all means open it up and take a wiff. :p I bought a BCM Socom upper and since it is my first carbine gas gun or this sort, do you think it would run ok with an H3 and sprinco blue that I have laying around?
Try just the H3 with the std spring first. That'll work with M193.
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m193 and 5.56 64gr Gold Dot is all I shoot. I think I have a standard BCM spring too.
and sprinco blue that I have laying around?
Use that as a Christmas tree ornament.
You mean you don't want all these Springco springs, AAC and LWRC stickers/Tshirts I have been saving for you?
Put them with the Obama/Biden swag.
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