Recently one of our members cut a barrel off and started having major problems. As we mentioned earlier, we would try to address some of the issues involved in a Thread or Sticky. Before we start let me remind you there is no exclusive answer to having an exact gas port size for each length of barrel. The reason being there are too many variables involved when you get down to the critical points. For example, as a general rule Military Ammunition is hotter (more pressure) than Commercial Ammunition which especialy affects the required Gas Port size. Believe it or not tolerances of the individual weapon itself can also effect the operation when it comes to Gas Ports. For example we would build 5 select fire rifles which are far more temperamental than the semi-rifle, made from the same components with the same Gas Port size and one we would have problems with and might have to adjust the Gas Port slightly to compensate. So with that said, you can see it is not an exact science selecting a specific Gas Port size. But we will give you some general sizes just for a starting reference. Keeping in mind the variances that may be required due to the above information. In addition in the case of cutting down a barrel you should never do it if it has a chrome lined barrel. Secondly you must keep in mind that the barrel will have the original Gas Port Hole size to match the original barrel length. So if you cut a 16in. Mid-Length Barrel down to 14" for example it would still have the original 16" Mid-Length Gas Port size. With that said, let me give you a general rule. *With exception of the barrels 11" and shorter which are the exceptions to the rule, The RULE is: *The longer the barrel or the farther the Gas Port is from the Chamber where the round is actualy fired the Larger the Gas Port Size required.
Some Examples to explain the logic: Keeping in mind they may very slightly due to ammunition and other factors as previously stated.
Barrel Length Gas Port Size
16" Car Length .063
16" Mid-Length .080-.085
Shorter Barrels Vary! Including 7" 10" 11".
Example is the 7" that may require a .065-.070
This would explain why if you do cut a barrel down you will have the larger gas port of the original barrel. If you cut a 16" Mid-Length down for example to 14.5" you would still have the .085 Gas Port when for the 14.5 you should have a .062.
OK! Now we have a problem! We cut the barrel down or we are just having a problem with our AR! The weapon is running too Hot as we call it which causes it to possibly run out of time! First of all if you have good Extractor and Ejector, I will almost guarantee that you will miss diagnose the problem in this case because you will start having a lot of double feeds and jams. But see nothing wrong with the ammunition, extractor, ejector, dirty chamber! This could likely be caused by too much pressure due to the incorrect size (now) of the Gas Port or the factory made an error. This allowing too much gas pressure to operate the weapon. How can we hopefully fix it? In most cases it can be fixed rather easily. BUFFERS!
Most standard AR-15 Rifles have the standard Buffer or Buffer Weight. There are also three other Buffers and Buffer Weights available. The H Buffer. The HH or (2-H) Buffer and the HHH Buffer or 3-H Buffer. Some times they are marked differently. But with the Standard Buffer being the lightest and the HHH or 3-H being the heaviest Buffer, we sometimes change Buffers to slow the weapon cycle down that is running too Hot. If you are experiencing this perceived problem in a rifle (Meaning it is running too Hot or Over-gassed) you want to move up slowly by increments in buffer weight one at a time. Starting with installing an H or H-1 Buffer first to see if it rectifies or decreases the problem. If it does not totally take care of the problem go the the next step the HH or H-2 Buffer and so forth. As a final solution there are also such a thing as a Gas Tube which has an adjustable manifold block on it that is adjustable by a Hex Screw.
Hopefully this will help explain Gas Ports and assist should you ever experience this problem in the future.