School me on what determines the AR gas port size

[Bret]

I'm very familiar with building AK's and their gas ports. Basically, most AK's are overgassed. The smaller of the hole in the barrel or the diameter of the gas port in the front sight determines the effective amount of gas that cycles through the AK. I was thinking that things work pretty much the same for an AR, but then noticed another potential variable when assembling an upper last night. An AR15 has a hole diameter in the barrel, a hole (or perhaps channel) through the gas block and a hole in the gas tube. I notice people talk a lot about gas port diameters (especially with SBR's), but I don't remember anyone ever talking about the hole size in the has tube. If an AR is overgassed, couldn't the hole size in the gas tube be used to effectively limit the amount of gas cycling through the system? To put it another way, why are people so concerned with the port size in the barrel when it seems to me that the hole diameter in the gas tube would be easier to control?

[Rogue556]

There are actually a few threads here already that dive pretty deep into restricting gas by means other than the port size in the barrel.

Gas tubes with undersized holes are already available through BRT. They also sell gas block inserts that do the same thing. With the kit they sell you tap the port hole on your gas block to accept the port insert and specify what port diameter you want when ordering.

I installed a .063 port into a Geissele gas block on a Colt 11.5" FBI barrel and it functions perfect suppressed and still cycles mil-spec pressure ammo unsuppressed with an H2 buffer.


https://www.m4carbine.net/showthread.php?206657-BRT-EzTune-Gas-Tubes

http://www.blackrivertactical.com/concrete5/store/#!/BRT-EZTUNE-Gas-Tube-Carbine/p/103167251/category=6464009

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[markm]

why are people so concerned with the port size in the barrel when it seems to me that the hole diameter in the gas tube would be easier to control?

Gas tubes are standardized items. If we gassed our guns by selecting from a bunch of different tube sizes, it would be a mess in my opinion. (users trying to figure out the correct tube, etc) Port size in the barrel should be sized right to match the barrel and gas length accordingly.

[vicious_cb]

I'm very familiar with building AK's and their gas ports. Basically, most AK's are overgassed. The smaller of the hole in the barrel or the diameter of the gas port in the front sight determines the effective amount of gas that cycles through the AK. I was thinking that things work pretty much the same for an AR, but then noticed another potential variable when assembling an upper last night. An AR15 has a hole diameter in the barrel, a hole (or perhaps channel) through the gas block and a hole in the gas tube. I notice people talk a lot about gas port diameters (especially with SBR's), but I don't remember anyone ever talking about the hole size in the has tube. If an AR is overgassed, couldn't the hole size in the gas tube be used to effectively limit the amount of gas cycling through the system? To put it another way, why are people so concerned with the port size in the barrel when it seems to me that the hole diameter in the gas tube would be easier to control?

Even if you restrict gas at the tube you just changing the volume going into the carrier not the pressure. Probably the most important aspect of the AR's gas system is the port pressure which only changes by where the port is in the barrel, ie closer to the chamber = higher port pressure.

[Renegade04]

Some of the factors in determining port size in a barrel may be barrel length, gas system (carbine, mid-length, or rifle), and barrel twist. Chambering may have some affect as well. A .223 REM or .223 Wylde may have a different port size than a comparable 5.56 chambered barrel due to the type of ammo used. I have seen a rifle-length gas system barrel (14.7") not do well with .223 REM ammo and do great with 5.56 NATO M193. This may be in part to have a slightly larger port opening. .223 REM ammo can be a bit under-powered and fail to cycle in some barrels with larger port openings. An example to note is that of Colt's Model 605. Initially, they used a full-length 20" M16 barrel (rifle-length gas system) of which they cut down to 15". They never made any compensation for the shorter barrel length and retained the same gas port size. This resulted in numerous cycling issues and the weapon was out for only a year before they scrapped it. Since then, others have figured out the issue and started opening up the gas port just slightly and it drastically improved the cycling. Mine works great.

[lysander]

Even if you restrict gas at the tube you just changing the volume going into the carrier not the pressure. Probably the most important aspect of the AR's gas system is the port pressure which only changes by where the port is in the barrel, ie closer to the chamber = higher port pressure.
Not quite.

There will be a considerable pressure drop across and orifice, the smaller the orifice, the larger the pressure drop. Besides the volume of gas is always the same the volume of the gas tube and the carrier cavity, but the mass varies varies, higher pressure in the same volume, i.e., more molecules of gas = more mass, and that is what determines the amount work done in the system.

Varying the port size has large effects on the pressure in the carrier cavity:
https://i.imgur.com/vggSOXv.png

[lysander]

.223 REM ammo can be a bit under-powered and fail to cycle in some barrels with larger port openings.

Got that backwards, the lower the port pressure, the larger the opening. That's why 300 Blackout subsonics need such huge ports compared to the supersonics (assuming no can).

The problem with the 605 is the short length of barrel beyond the port. With the 605, no sooner is the gas port uncovered by the bullet, the bullet leaves the barrel and all that barrel pressure is uncorked and blows away, thus the mass of the gas in the system (gas tube and carrier) is insufficient to produce the work required. The larger gas port in the 605 is to allows the mass of the gas forced into the system to be greater by virtue of the higher pressure in the gas tube from the larger port.

[lysander]

To put it another way, why are people so concerned with the port size in the barrel when it seems to me that the hole diameter in the gas tube would be easier to control?
The gas tube is (usually) made from standard stainless steel seamless tubing. Buy it by the ton, it's cheap as dirt. Even the fancy Titanium and INCONEL tubes are made from standard seamless tubing used in the aircraft industry, and not that uncommon.

The gas port is a hole drilled in a barrel.

Why do you think that the inside diameter of the tube would be easier to vary and/or control?