View Full Version : Gas port location thoughts
I was looking at this chart and considering the difference in port size:
http://www.tacticalmachining.com/learn/ar-style-rifles/ar-15-gas-port-sizes.html
The difference in size between the largest port on the 20" and the smallest used on a 16" carbine gas system is about 5:2 when you realize that the real size of the port is the area of circle the hole describes, which is a square of the radius.
Port size is a proven way to make up for huge increases in barrel dwell, but smaller holes wear faster and are likely to cause fouling as the gas slows on the other side of the hole. So it isn't awesome.
With modern rifles that don't rely on the location of the front site for the gas port, why continue to use 7" and 9" gas tubes for shorter barrels? Looking that the port sizes for the 11.5" and 20" guns, I think you could probably keep closest to a large, healthy .095" port size across the board if you just copy the proportions of the 20" barrel with its 7.5" front section, which is a 37.5% of barrel length. The 11.5" barrel has 4.5" in front of the port, which gives it a similar 39% proportion and almost as large a port.
While the pressure drops off on a curve, so does barrel dwell with added barrel length, so there is some cancelation that flattens the curves. Which is why I'm guessing that a 16" with the port 6" from the muzzle will also allow the use of a .090+ port size, or 5.4" on a 14.5" barrel. Clearly, trial and error testing of carrier speed would yield the best results, as would settling for a single buffer weight. But if I was buying a custom barrel to go under a rail I would port it in that proportion.
There's no reason to keep basing gas system length on bayonets and handguards designed for 10.5" barreled Vietnam weapons. Thoughts?
Black River Tactical
Knights Armament Co
Noveske
These all seem to agree with you, at least regarding alternative gas port locations.
everready73
11-20-18, 08:04
Check out Black River Tactical. They have some new stuff out that is pretty innovative IMO
The current 3 gas port lengths are all fine when ported correctly in all the reasonable barrel lengths/dwells. I find the only thing lacking is the imbeciles who continue to spec ports ridiculously large.
Gas port location is more about gas pressure than it is about "dwell time".
Gas port location is more about gas pressure than it is about "dwell time".
Dwell time of the bullet past the port determines the total amount of gas that will be fed into the tube, given equal port size.
Dwell time of the bullet past the port determines the total amount of gas that will be fed into the tube, given equal port size.
Nevertheless, gas port location is about the pressure, the so-called "dwell time". Gas pressure at the port is far more important than the distance from port to muzzle.
The distance from gas port to muzzle is an inch and a half longer in the Colt 16 inch barrel than the Colt 14.5 inch barrel. Yet, the extra "dwell time" isn't enough for Colt to change the port diameter.
The difference between the carbine gas and the mid gas system is two inches. The pressure drops enough that there is a significant difference in port diameter. Colt uses a .0625 inch gas port in the carbine gas system. I believe a .072 inch gas port is used for the middy.
Location is about port pressure. Not "dwell time".
"Dwell time", the time the bullet is in the bare from port to muzzle is almost meaningless. The difference in time between a14.5 inch and 16 inch barrel is like one millionth of a second.
vicious_cb
11-20-18, 23:20
Mistwolf is right. The closer the gas port to the chamber, the higher the port pressure. The trick is trying to tap the gasses as far enough from the chamber as possible so that you arent causing early unlocking and excessive carrier velocity but keeping enough barrel in front of the port to pressurize the system long enough to run the action.
Nevertheless, gas port location is about the pressure, the so-called "dwell time". Gas pressure at the port is far more important than the distance from port to muzzle.
The distance from gas port to muzzle is an inch and a half longer in the Colt 16 inch barrel than the Colt 14.5 inch barrel. Yet, the extra "dwell time" isn't enough for Colt to change the port diameter.
The difference between the carbine gas and the mid gas system is two inches. The pressure drops enough that there is a significant difference in port diameter. Colt uses a .0625 inch gas port in the carbine gas system. I believe a .072 inch gas port is used for the middy.
Location is about port pressure. Not "dwell time".
"Dwell time", the time the bullet is in the bare from port to muzzle is almost meaningless. The difference in time between a14.5 inch and 16 inch barrel is like one millionth of a second.
Well, that's Colt - one manufacturer. But I already included a list of common port sizes used in different barrel/gas tube combinations, and 16" carbines have as little as .0625" ports while 14.5" barrels go as large as .086". The fact that Colt uses a really tiny port for the M4 doesn't change the fact that the number of Joules of Work that acts on the carrier is the product of both pressure (force) and duration. Duration comes from how long the bullet remains in the bore. An extra 1.5" of bore is a 20% increase in the amount of time the gas system is pressurized, which means 20% more acceleration on the carrier.
The other factor in all of this is that the small ports in short gas systems erode faster, so a small port may represent the minimum port that functions but lasts the life of the barrel, rather than the ideal port size. Which is something the military might be more concerned about than a company selling range rifles.
Rayrevolver
11-21-18, 07:51
These are great technical discussions but there is a lot information we don't know about when it comes to timing. I would like to see a timeline of when the bolt starts to unlock, when the gas tube is clear of the carrier key (and vents), bullet uncork etc.
You can find a report called "Comparison of a theoretical and experimental study of the gas system in the M16A1 rifle" August 1971, BRL Report 1548, somewhere on the internet. Lots of good background on the firing sequence and things they tested.
I started this thread below which did have some good discussion about port size and dwell:
https://www.m4carbine.net/showthread.php?173315-11-5%94-Mid-Length-Barrel-Discussion
I want to say Tom12.7 mentions a +1" gas system on the 11.5 as being beneficial. He is a good resource for these discussions.
For me, I would be interested in why gas trap systems went out of vogue. The Vltor FOG system was similar and it never went anywhere. I had one of these uppers and sold it before my suppressor came in... kicking myself!
This a quote one can find when searching "dwell time in AR15" it's from Paul @ BCM
QUOTE
Why BCM chose the 11.5" SBR over the 10.5"
I get this question a lot. It is a good one. When we spec our program we can build anything we would like, so we sat down and looked at the pros and cons.
First Statement: I know a good 10.5" SBR can run very well. I personally own a semi MK18 type and it runs great. No problems.
A: Dwell time.
Dwell time is the time that your gas operated weapon maintains pressure to continue the cycling of the weapon. It primarily exsists from the time the bullet passes the gas port in the barrel to the time the bullet exits the muzzle. When you pull the trigger and fire the weapon the movement of the bolt carrier group unlocks the bolt, extracts, and ejects the spent casing. Then it cocks the weapon, feeds, chambers the next round, and then locks again. One of the thing that can make SBRs finicky is the dwell time (or lack of).
The 11.5" carbine is approximately 4% longer weapon than its' 10.5" counterpart, but this extra inch gives the barrel a 40% increase in length for dwell time. IMHO, this is an excellent trade off. This additional dwell time (all other things being equal) will allow the carbine to be more forgiving to different ammo types, extremes in air temperature, weak or worn extractor springs, worn extractors, buffer weights, etc.
Last Statement: For those folks who have a 10.5" that works very well, I would reply; me too. (see first statement)
If I had to "go to war" with a AR15 Carbine, I would grab the BCM 11.5".
The BCM 11.5" Runs Like a Dream.
Hope this info helps,
Paul
These are great technical discussions but there is a lot information we don't know about when it comes to timing. I would like to see a timeline of when the bolt starts to unlock, when the gas tube is clear of the carrier key (and vents), bullet uncork etc.
You can find a report called "Comparison of a theoretical and experimental study of the gas system in the M16A1 rifle" August 1971, BRL Report 1548, somewhere on the internet. Lots of good background on the firing sequence and things they tested.
I started this thread below which did have some good discussion about port size and dwell:
https://www.m4carbine.net/showthread.php?173315-11-5%94-Mid-Length-Barrel-Discussion
I want to say Tom12.7 mentions a +1" gas system on the 11.5 as being beneficial. He is a good resource for these discussions.
For me, I would be interested in why gas trap systems went out of vogue. The Vltor FOG system was similar and it never went anywhere. I had one of these uppers and sold it before my suppressor came in... kicking myself!
Dwell time can't really be directly compared for different gas system lengths because the port pressure is different. The balancing act is similar to calculating recoil/bullet inertia - velocity x mass. You can maintain the same inertia by increasing velocity while decreasing mass. That's like how you need a shorter dwell to make up for a higher port pressure. So in the thread you started, you point out that the 16M has nearly the same dwell as the 20R - which means that the impulse is going to be larger from the 16M because it has the same dwell but more pressure. The ideal would be less dwell than the 20R to make up for the higher pressure.
Rayrevolver
11-22-18, 22:00
Dwell time can't really be directly compared for different gas system lengths because the port pressure is different. The balancing act is similar to calculating recoil/bullet inertia - velocity x mass. You can maintain the same inertia by increasing velocity while decreasing mass. That's like how you need a shorter dwell to make up for a higher port pressure. So in the thread you started, you point out that the 16M has nearly the same dwell as the 20R - which means that the impulse is going to be larger from the 16M because it has the same dwell but more pressure. The ideal would be less dwell than the 20R to make up for the higher pressure.
Did you read through the whole 11.5M thread or just my first post?
Its not about the port pressure but the gas flow/total volume of gas. Remember the size of the port and the pressure will determine the total volume of gas delivered to the BCG during the dwell time (and blow down).
A 16M with the same dwell as a 20R, but obviously a smaller port, means a similar gas drive. The 16M will start to unlock earlier than the 20R, so that means it is unlocking at a higher chamber pressure, which requires more energy.
In my mind, the dwell time for the 20R is the benchmark so you should absolutely try to compare to it. You start to reduce that time and you reduce the time you have available to overcome things like a dirty system, lack of lube, debris, manufacturing tolerances, system drag, low powered ammo etc.
So no, its not as easy as increasing pressures and reducing dwell. There is more to it than simple math and engineering.
Dwell time is the time that your gas operated weapon maintains pressure to continue the cycling of the weapon. It primarily exsists from the time the bullet passes the gas port in the barrel to the time the bullet exits the muzzle.
Dwell time is short. A tiny fraction of a second short. Too short to have any affect on the pressure entering the gas system. There's a chart that's been posted over and over again, showing just what a small fraction of a second dwell time is. It's so short, that the bullet has exited the muzzle before the gas can positive the system. The system operates strictly off of the residual pressure remaining in the bore after the bullet has left the barrel.
If the pressure operated the action before the bullet left the muzzle, as longer dwell time would make no difference. Adding a suppressor would make no difference. Why? Because the bullet would be in the same place in the bore the action operated with each shot. The action would operate before the bullet (and the gas behind it) reached the suppressor.
The length of the barrel after the gas port determines how quick the gasses in the bore vents and pressure drops to atmospheric.
Did you read through the whole 11.5M thread or just my first post?
Its not about the port pressure but the gas flow/total volume of gas. Remember the size of the port and the pressure will determine the total volume of gas delivered to the BCG during the dwell time (and blow down).
A 16M with the same dwell as a 20R, but obviously a smaller port, means a similar gas drive. The 16M will start to unlock earlier than the 20R, so that means it is unlocking at a higher chamber pressure, which requires more energy.
In my mind, the dwell time for the 20R is the benchmark so you should absolutely try to compare to it. You start to reduce that time and you reduce the time you have available to overcome things like a dirty system, lack of lube, debris, manufacturing tolerances, system drag, low powered ammo etc.
So no, its not as easy as increasing pressures and reducing dwell. There is more to it than simple math and engineering.
I think you didn't understand the OP. My point was that that "total volume of gas" is the product of three dependant variables - pressure, port size and dwell - and that it would be best to adjust the port location rather than the port size.
So while you want to keep the 20R dwell time, I'd rather see the port location adjusted so a 20R port size could be utilized. Moving the port forward decreases pressures and uses shorter dwell to decrease gas volume rather than a restrictive port size that will wear over time, while dwell will never change with wear.
I think a big port is "healthier" for the rifle's function, so I would change the port location/dwell to get the correct volume. You would change the port size to get a particular dwell, but you haven't explained why you think dwell is important in and of itself - especially when it requires the use of small gas ports which seems to lead to haphazard results when you change ammo.
Again, port location has a greater impact on gas drive than "dwell time". Port location has a much greater impact on port diameter than "dwell time". How much gas is generated has a greater impact on gas port location than "dwell time". Bore diameter has a greater impact on gas drive than "dwell time".
What we are calling "dwell time" is really blow down time. The shorter the distance from gas port to muzzle, the less time it takes for pressure in the bore to drop to atmospheric at the port location.
Again, port location has a greater impact on gas drive than "dwell time". Port location has a much greater impact on port diameter than "dwell time". How much gas is generated has a greater impact on gas port location than "dwell time". Bore diameter has a greater impact on gas drive than "dwell time".
What we are calling "dwell time" is really blow down time. The shorter the distance from gas port to muzzle, the less time it takes for pressure in the bore to drop to atmospheric at the port location.
What do you mean it has a "greater impact"? Size, location and duration all have a major effect on the volume of gas that gets to the carrier. Too little barrel after the port and it won't matter how big the port is - the gun won't cycle. I don't see how you can discount any one of the three factors that pressurize the gas system.
I'll just leave this here...be sure to read the description. A rendition of the (never issued) Colt 605.
https://www.brownells.com/firearms/rifles/semi-auto/brn-605-5-56-prod124180.aspx
Chamber pressure has a curve during firing.
This curve includes both the time the projectile is in the bore and time it takes for residual pressure to blow down after the projectile has exited the bore.
The total impulse or area under the curve determines the ability of the pressure to do work, such as accelerate the projectile or cycle the action.
The area under the curve is really a summation (integration) of the pressure at each instant, or to state it simply, PRESSURE X TIME.
In a straight blowback weapon, the entire pressure curve is used to power the action.
In a gas operated, locked breech weapon, placement of the gas port relative to the muzzle determines which later portion of the pressure curve is used to power the action.
A port placed at the muzzle (i.e. gas trap) uses only residual pressure, while a port placed at the case mouth uses the entire curve.
Obviously, a port placed somewhere in the middle uses uses a combination.
Once a portion of the curve is selected by the port placement ( the Available Gas Drive ), the port size scales that down to the Actual Gas Drive used to power the action.
There are many combinations of port placement and size that can all yield the same total number for Gas Drive at the front end of the system.
If this was a simple system, you could select any of these combinations and they would produce identical results at the back end of the system.
But it's not, and they don't.
Sparky5019
11-07-23, 10:48
Chamber pressure has a curve during firing.
This curve includes both the time the projectile is in the bore and time it takes for residual pressure to blow down after the projectile has exited the bore.
The total impulse or area under the curve determines the ability of the pressure to do work, such as accelerate the projectile or cycle the action.
The area under the curve is really a summation (integration) of the pressure at each instant, or to state it simply, PRESSURE X TIME.
In a straight blowback weapon, the entire pressure curve is used to power the action.
In a gas operated, locked breech weapon, placement of the gas port relative to the muzzle determines which later portion of the pressure curve is used to power the action.
A port placed at the muzzle (i.e. gas trap) uses only residual pressure, while a port placed at the case mouth uses the entire curve.
Obviously, a port placed somewhere in the middle uses uses a combination.
Once a portion of the curve is selected by the port placement ( the Available Gas Drive ), the port size scales that down to the Actual Gas Drive used to power the action.
There are many combinations of port placement and size that can all yield the same total number for Gas Drive at the front end of the system.
If this was a simple system, you could select any of these combinations and they would produce identical results at the back end of the system.
But it's not, and they don't.
So I’ll resurrect this to ask…
What is the minimal distance from port to muzzle that is reliable? I’ve seen various Dissipator configs with seemingly different lengths past the port.
Does this change based on gas system length or port size?
I'm waiting for dual gas ports to come into fashion. If done correctly, I can see it working, though honestly I'm not sure it would ever give up enough benefit to be worthwhile.
So I’ll resurrect this to ask…
What is the minimal distance from port to muzzle that is reliable? I’ve seen various Dissipator configs with seemingly different lengths past the port.
Does this change based on gas system length or port size?
Yes, it changes as the gas system length increases because the pressure at the gas port drops significantly as it gets further from the chamber.
Sparky5019
11-07-23, 12:46
Yes, it changes as the gas system length increases because the pressure at the gas port drops significantly as it gets further from the chamber.
That is expected. Do expound…
That was purpose in this resurrection…no one has answered the first question. Lol
That is expected. Do expound…
That was purpose in this resurrection…no one has answered the first question. Lol
Well the port to muzzle would also depend on overall barrel length too since the pressure drops with length. I've seen a rifle system cut to 16" with the port opened up to .090". Anything needing a larger port than that is getting silly to me.
Sparky5019
11-07-23, 13:05
Well the port to muzzle would also depend on overall barrel length too since the pressure drops with length. I've seen a rifle system cut to 16" with the port opened up to .090". Anything needing a larger port than that is getting silly to me.
Like I was saying; minimum port to muzzle length for each gas system. We all know there are a lot of factors but keeping the back end of the gun and ammo to a relative constant because they are the easiest to adjust if needed.
It’s an interesting discussion.
It’s an interesting discussion.
I like staying in the center of the fairway and not pushing it too much. The interesting thing is how badly "they" screwed up the carbine gas system shorties with oversized ports. O-rings and H3 buffers and all that nonsense to fix stupidity.
Off the top of my head, I'd guess that the 10.3 Colts have one of the shortest port to muzzle lengths. But when you don't let a moron spec the port too big, they run just fine.
18” rifle, 14” mid, 10.3” carbine.
Yes some examples of shorter will run, but not with a reliable margin in my opinion.
18” rifle, 14” mid, 10.3” carbine.
Yes some examples of shorter will run, but not with a reliable margin in my opinion.
I agree. Like I mentioned, a guy I know from the Sheriff's Office Armory took an extra Colt Rifle upper and cut it to 16". His worked, but I wouldn't be comfortable with that for serious use.
titsonritz
11-26-23, 23:27
18” rifle, 14” mid, 10.3” carbine.
Yes some examples of shorter will run, but not with a reliable margin in my opinion.
Yep, properly ported those are the realistic numbers, saw a thread here recently that was akin to pissing in the wind. Lots of knowledgeable folks been doing shit a long time have been telling us what is what, I believe them.
I'm waiting for dual gas ports to come into fashion. If done correctly, I can see it working, though honestly I'm not sure it would ever give up enough benefit to be worthwhile.
Bill G already has three phased gas ports in one of his newer short barreled rifles. I haven't seen the gas tube setup but supposedly it makes a short barrel (9" if I recall correctly) more reliable.
I learned quite a lot while watching this livestream. The phased array gas port topic comes up at around 1:01:35
Some of the key points for those who won't watch.
- .MIL (SOF I assume) is using 6ARC
- G recently won a contract for a precision rifle
- 3 phase gas port for short barrel reliability
- He is a talker and very proud of his products
- Sulfer can be added to barrel steel to increase machineability
- The El Jefe rifle was produced for an LE contract and looks kind of cool at that price point
https://youtu.be/8_Ktu4GIO5U?si=x3l9PPtB1GQ50bDQ
This video is a lot more than a new product introduction and is worth the time investment to watch.
BufordTJustice
12-01-23, 15:10
Bill G already has three phased gas ports in one of his newer short barreled rifles. I haven't seen the gas tube setup but supposedly it makes a short barrel (9" if I recall correctly) more reliable.
I learned quite a lot while watching this livestream. The phased array gas port topic comes up at around 1:01:35
Some of the key points for those who won't watch.
- .MIL (SOF I assume) is using 6ARC
- G recently won a contract for a precision rifle
- 3 phase gas port for short barrel reliability
- He is a talker and very proud of his products
- Sulfer can be added to barrel steel to increase machineability
- The El Jefe rifle was produced for an LE contract and looks kind of cool at that price point
https://youtu.be/8_Ktu4GIO5U?si=x3l9PPtB1GQ50bDQ
This video is a lot more than a new product introduction and is worth the time investment to watch.
The GA barrels are also using extended gas system lengths. Not unlike KAC and Noveske.
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The GA barrels are also using extended gas system lengths. Not unlike KAC and Noveske.
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And BRT
BufordTJustice
12-01-23, 19:49
And BRT
Yep. Bought their first run 16" intermediate gas. [emoji41]🤌[emoji106]
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Yep. Bought their first run 16" intermediate gas. [emoji41]🤌[emoji106]
Sent from my iPhone using Tapatalk
What difference do you notice between that a carbine and Middie?
BufordTJustice
12-01-23, 22:42
What difference do you notice between that a carbine and Middie?
It shoots smoother than the old school Colt 6920's, or a BCM 16" middy. Which isn't really a huge deal.
What *is* a big deal, is that it works 100% with steel case ammo and PMC Bronze. And it does that with a smaller delta between the recoil impulse of the weak stuff compared to Frontier 62gr or Federal M855. Not unlike my KAC mod2 16" upper. Very similar to that.
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