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?
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