Gas port location thoughts

[Gödel]

I was looking at this chart and considering the difference in port size:
http://www.tacticalmachining.com/lea...ort-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?

[JediGuy]

Black River Tactical
Knights Armament Co
Noveske

These all seem to agree with you, at least regarding alternative gas port locations.

[everready73]

Check out Black River Tactical. They have some new stuff out that is pretty innovative IMO

[markm]

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.

[MistWolf]

Gas port location is more about gas pressure than it is about "dwell time".

[Gödel]

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.

[MistWolf]

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]

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.

[Gödel]

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]

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...rel-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!