Expressed as a percentage, a MK18 gas port (0.070) is how much larger than an LE6920 gas port (0.062)?
Hint: It’s not 13%!
Assuming gas ports are perfectly circular and flat (as opposed to on a cylinder:
0.070" diameter = 0.003848451 in^2
0.062" diameter = 0.00301907054 in^2
(pi * r^2)
0.003848451 / 0.00301907054 = 1.27471384
27.47% more area in a 0.070" gas port than a 0.062" gas port.
13% more than 0.062 would be 0.00341154971 in^2 or 0.0659" diameter.
But that’s both a) a flat circle and b) a circle.
So are gas ports perfectly circular (on a cylinder) and why do you ask?
Bingo…
I just ran across an article by a well known instructor who claimed the difference was 13%. My math came up to 27.4%. I just wanted to make sure my calculator was working correctly.
I’m going to say a gas port is as close to circular as you can get if a reamer is used to finish them v. a drill.
I’m guessing that the 13% figure is from the fact that that the number .070 is literally 13% larger than .062.
(070 - .062)/.062=0.129
What probably happened is whatever instructor you heard that figure from did not take into account that increasing a circle’s diameter does not increase the area an equal percentage.
One thing for sure is that my head hurts now.
Wait till you try to figure out how much more the larger hole flows
Now you’re going way beyond 10th grade geometry… To do this right you’d need some kind of flow bench, or some pretty fancy computer modelling.
Bimmer
Wrong…
what would be the ‘right’ way?
I’m sure you can get a pretty close approximation with some accurate numbers about the cartridge being fired and some somewhat “basic” physics. It wouldn’t be especially simple physics, though–college level stuff but no PhD stuff, I imagine. I don’t think you’d need any fancy modeling to get pretty good numbers.
Either way, you’re going to need some tools far beyond the technological capabilities of the word “Wrong.”
Unless, of course, you just want to be a smart-ass.
10th grade trig guys - at the latest. Unless you are a stickler…most probably dont care, but nice dispell of the simple ratio increase.
The barrel is a cylinder, the gas port is (presumably) cut with a cylindrical bit ONTO a cylinder (making it like an oval sticker on a beer can, not a circular sticker–therefore the gas port can’t actually be described as perfectly circular), and then you’ve got all sorts of gas flow and pressures and all that stuff.
I fail to see how triangles have anything to do with this.
And the point is that (for fun and amusement) we’re being sticklers. As I said, this isn’t PhD-level stuff, but it sure as hell is way beyond high school.
A petro-engineer buddy of mine adds, “That and simplistically its still a flowrate problem. Less simplistically its an extremely complicated gas expansion and flowrate problem.”
I was going after trig unit of circle - you are right. Going back loading magazines for tomorrows post July 4th shooting.
I think the real bitch of this problem is that the actual question (if you want to get real down and dirty with it) if not “how much bigger is one gas port over the other?” but “what is the difference in force acting on the BCG given different gas port sizes?”
Because if you only need x lbs of force through the gas port (and down the gas tube) to properly cycle the BCG, then reducing a gas port’s size on a shorter barrel by y% is going to reduce the force on the BCG by more or less than y% depending on the math. On the engineering level (we’re way way past anything any shooter would ever need to know), you’ve got factors like friction of the smaller gas port/tube acting on the gas, the higher pressure or heat of the gas flowing down a smaller channel, etc. etc. A smaller or larger channel for the gas to flow to the BCG might have a disproportionate (either amplifying or moderating) effect on the pressures involved
In my buddy’s words: “I don’t remember my fluid dynamics that well. But a larger port OD [outer diameter] will result in a slightly smaller pressure inside the port. Then the expansion to the BCG will be slightly smaller. That would be the governing aspect.”
Essentially, it’s a fluid dynamics problem more than a geometry problem. The geometry’s just the crust holding this ugly pie together. A pie I am neither interested nor equipped to tackle.
Edit: He adds, “Again, in reality the numbers we’re talking about here are miniscule enough to be insignificant.”
well, he IS a petro-engineer :p, but we’ll forgive him.
definitely not insignificant, for our purposes. as most of us know from all the gas port size discussions, a few thousandths in gas port diameter can make the difference between a functioning and non-functioning rifle. correct gas port size is one of the most important aspects of the operating system. look at the SCAR barrel chop thread - cutting down the barrel from 16" to 10.5" resulted in their weapons only functioning with suppressors, so those guys have some gas port work to do.
trig just gives us the different in port size areas, based on the diameter. to figure out the flow and pressures, it’s definitely going past basic college physics. and a regular flow bench isn’t going to do much good (unless it measures supersonic flow).
i’d guess that it’s also no longer a fluid dynamics/mechanics problem because fluid dynamics usually deals with incompressible flow. i’m thinking it’s more of a gas dynamics (compressible fluid mechanics/thermo) problem as the flow is supersonic, which makes it a bit more complicated.
+1
But it is funny to see guys arguing about this.
whatya expect from a geek? (speaking for myself only). 
technical/scientific questions are fun to mull over.
however, from a practical standpoint, i do wonder how many manufacturers perform calculations/analysis to determine correct port size vs. trial and error (trying out different diameters and seeing which one works).
I am right there with you in terms of geekdom. I just did not go far enough in math classes to be able to follow along the technical details and I did not take more than a few semesters of (honors) level physics and no engineering classes (I was software). And it is funny to see a bunch of what I presume are middle aged engineers or at least did a bunch of that 20-30 years ago in college arguing on how to approach this subject.
It is probably something someone who is making money in the business selling rifles or uppers / barrels would like or should like to know, though a lot of that sort of people are probably more practical engineers and less of a schooled engineer. (Ie, guys designing rifles probably did not study fluid or gas dynamics but rather have a long history with the guns themselves and do it more seat of pants – just a guess and not meant to put anyone down).
This is almost as entertaining as the TOS thread a few years ago about whether an airplane could take off if the runway rolled along opposite the direction of travel at exactly the same speed as the airplane was, instantaneously correcting to always exactly match the airplane speed. (Though the guys who thought they were so smart etc really were dumb on that one and you did not need complex dynamics to answer it – the thread itself was wildly entertaining with all the wannabe physicists and engineers and mathematicians answering it wrong).
That thread must have gotten around. My son saw it somewhere and consulted with me about an answer. The question is like asking “How many of each kind of animal did Moses put on the ark?”