Over on another site there is a thread asking for the correct gas port size for a 16" barrel with a mid-length gas system.
Multiple replies mention barrel profile/OD at the gas port advising that a barrel that uses a .750" gas block would need a different size port than a barrel using a .625" gas block.
You are talking about is a .0625" difference in the length of the port. I do not see how this would make a difference.
Am I blind or are these people full of it?
Orifice length DOES have an impact on how much flow can get through it. The shorter the orifice length, the smaller the port needs to be.
So yes, barrel thickness will have an effect on the gas port size.
I’m going to school for fluid power and orifices and how they affect flow is a vital part of properly designing a system.
If you want me too i can dig around my notes and find the math to prove it.
The larger the gas block seat the larger the gas port needs to be. .625" gas block seats and .750" seats are pretty close to the same size. .936" gas block seats have to have larger gas ports.
The distance from the bore to the gas tube is EXACTLY the same regardless of barrel size. What’s the difference between 0.750 and 0.936? Your right, 0.186 of an inch. Half of that would be 0.093. When you increase the “wall thickness” (your words) by 0.093, you have to increase the bore of the gas block. While the length of the gas port in the barrel is increased, the length of the plenum in the gas block is decreased proportionately. How do you like them apples?
Yes, everybody is correct.
It does matter, but not very much.
A standard .750" gas block journal has a gas port length of .263"
A .625" gas block journal has a gas port length of .200"
Our insertable gas ports are very short ~.150" or .075" and seem to only run about .003" smaller than a gas port directly in the barrel.
I’m not sure what I did to set you off with the name calling but whatever I did, I’m sorry.
While the distance from the center of the bore to the gas tube is the same it’s the initial restriction of the port in the barrel that’s important. The hole in the gas block and gas tube are way larger than the port itself, so its effect on flow shouldn’t (assuming everything is drilled right) be noticeable.
I will have the math tomorrow if time allows.
I hope this doesn’t come off sounding snotty or rude, because I’m not trying to, but if I’m wrong the why are the gas ports being drilled to different sizes? I know it’s not by much but with the amount of pressure and they already small gas port size it doesn’t take much to have an effect.
Well, to clear something up, he did call you “schoolboy” as an insult… Soooo…
Yes, of course you are right, it does matter and they are in practice different sizes for different diameters. I thought is was completely clear and easy to grasp by exaggerating the issues; by imagining a 12" diameter gas block for instance. It makes sense that if the port is the smallest size you are working with that the diameter changes that port’s cross sectional length changes also. Longer the port, the bigger it would need to be to stay the same pressure and volume.
Some sources:
http://www.mcnallyinstitute.com/13-html/13-12.htm
https://www.m4carbine.net/showthread.php?t=41592
I know that these two equations I gave below do not list wall thickness but if you can see in the pictures in the link below how the tapered orifice slightly reduces the wall thickness, it can maybe lend some validity to my post.
Sharped edge orifice: Q=24.12*(orifice area)*√(psid)
Angled edge orifice: : Q=(24.12*(orifice area)*√(psid))*1.5
Oh!!! No he didn’t, “schoolboy” just times square rooted that motherfucker! 
Yeah, I have nothing to offer, but I’m VERY interested in an intelligent discussion on this topic.
Hoping it doesn’t get locked
Robb - thanks for the info. What would be an example of the sizing differences you have seen?
My notes are at work.
I’m interested in this topic also. It’s great to see an engineering student getting quantitative about our favorite machine.
Unfortunately I see some difficulties in applying fluid power equations to this problem.
First of all, I haven’t seen any sources that can give the viscosity of the combustion products in the bore. This may be temperature dependent, and I can’t find any temperature profiles (not just max temperature) for the gas travelling down the bore.
Secondly, the flow through the gas port may be supersonic.
Third, the flow is a dusty flow, with powder ash and partially burned grains travelling with it.
The best approach may be empirical. The only experiment I can think of is to progressively shorten the gas port of an existing barrel by milling a .125 concentric hole in the barrel surface at the gas port (.125 is the diameter of the vertical hole in the FSB, iirc). Then we could look at the change in average bcg velocity.
If these kinds of simple, inexpensive studies have been performed by the industry, I can’t find them.
I personally intend to do a study in which the gas port is reamed progressive larger, and the buffer weight is increased to give the same bcg retraction. This would be a a useful correlation to know.
StainlessSlide- You are right about applying my two given equations to the “fluid” (gases are technically fluids) that is used to operate an AR. The two given equations are basically dumbed up versions that my instructor gave us, they are meant to be used in the field for a quick estimate on a hydraulic system. The equations lack the option for fluid viscosity, temp, specific weight, orifice length, etc. If a similar equation was to be applied to a compressible gas, a compressibility factor would need to be added to the equations, and like you said taking in to account the fact that the gas is probably traveling at speeds greater than what is seen in your average pneumatic system. (Approx. 1130 fps or Mach 1 in an 1/8 inch pilot line)
I gave the equations and my sources just to show that it is a real thing for any who had their doubts.
A test, like the one you stated, would be very interesting.
The differences between the equations for flow through a sharp/chamfered/rounded edge orifice address the energy loss due to the type of entrance/exit as well as the length of the orifice. The K value, which is experimentally determined, accounts for those energy losses. There is an incredible amount or energy loss with a sharp edged orifice. The flow forms a vena contracta that can effectively cut the flow area down by a factor of 4.
http://en.wikipedia.org/wiki/Vena_contracta
Funny thing is, looking at these k values and using the equation Q=AVK, it looks like if A and V stay constant a thicker orifice will yield a higher flow rate (less loss).
When the orifice length is less than 1/4 the orifice DIA, the K value is 0.62
When the orifice length is 2-3 times the orifice DIA, the K value is 0.82
(Thanks to RideTheLightning for the link)
I recall an aritcle discussing volumetric efficiency of internal combustion engines. The article stated that because gasses are compressible, they will not flow through an orifice faster than mach 1.
Compressiblity/density changes aside, the only reason I see longer ports needing to be bigger is to counter the frictional losses through a longer orifice. We’re only talking 0.063" additional length for 0.625"/0.75" fsb journals so I can’t imagine the losses are that great. The port on a 0.936" journal probably sees a more substantial loss.
There is no hard rule on how much larger a port must be for a given diameter of barrel.
It’s different at each configuration of gas system, cartridge etc…
Something people don’t think much about is the expansion ratio of the gas system.
The smaller the port, the more differential between gas tube diameter and port diameter.
The more differential you have, the more you need to expand the gas in the gas tube before you can push the carrier.
The longer the gas tube, the more expansion.
The pressure supply in the barrel is lower the longer you move the port from the chamber.
The larger the gas volume behind the bullet when it passes the port, the less pressure drop you get due to bore expansion during bullet travel to the muzzle etc.
This stuff is tricky to model with so many active variables…
I have done a lot of port size development in my years of making custom AR barrels.
I could not put my experience into a math formula, but I can now call out a correct gas port size when given all the components of the gun…
There is also the fact the gases do not pressurize the BCG to operating levels until after the bullet has exited the barrel
They actually pressurize and just barely start moving the carrier before the bullet exits, but the bolt does not unlock until after the bullet exits.
This is all by design and critical to extraction of the fired cases.
As soon as the carrier moves back about 1/8", the exhaust ports open up and the carrier continues only on kinetic energy.
The unlocking starts at about 1/4" of rearward movement.
Eric D. - that source I used for the pictures. I’ve used the Q=VA quite extensively and adding that k value seemed kind of strange. (As Q= flow rate. A= conductor (pipe) area. V= velocity of fluid.)
In industrial applications it is common practice to use a long line of tubing coiled up as a flow control, the longer the long the more restrictive, so that whole k value thing seems weird and I have never seen it used like that.
Ar15barrels- If all variables are kept the same with the exception of the gas port length the gas port will be smaller for a smaller diameter barrel, at least that’s what my findings and math have told me. I’m not, by any means, second guessing your experience.
StainlessSlide- I’m VERY interested in how this turns out. It would be interesting, I think, to keep the buffer weights the same to see how much it takes before over gassing becomes an issue, and how much faster the BCG gets.
This.
The minor loss K values for the orifice characteristics are almost useless when comparing a flow that is this turbulent, and that has such a ridiculously high Reynolds number.
Also, even if the flow was not approaching supersonic, those K values are for incompressible fluid flow. The gases in the chamber are definitely not incompressible.
So unless you had some batshit crazy custom CFD software hidden up your sleeve, modeling this behavior mathematically with any degree of accuracy would be VERY hard.