Gas Port AR
AR-15 Gas System Parts
The Gas Port
The gas system starts at the port, a small hole drilled directly into the barrel. The port allows gas produced inside the barrel to escape, to be vented into the gas block.
Gas Port Diameters
Gas port diameter simply measures how wide the gas port is on the barrel. A collection of measurements from different manufacturers and barrel makers illustrates that port diameters tend to fall within certain ranges. These are the most common ranges of diameters you'll find on AR-15 barrels measuring 11.5" to 24" in length. Port diameter also changes depending on the thickness of the barrel itself (0.625", 0.750", or 0.825"):
AR-15 GAS PORT SIZES
AR-15 Platform 5.56mm
20" rifle 5.56 0.0935
20" rifle 5.56 0.096
20" rifle 5.56 0.098
16" mid 5.56 0.078
16" mid 5.56 0.081
16" car 5.56 0.0625
16" car 5.56 0.065
16" car 5.56 0.07
14.5" car 5.56 0.067
14.5" car 5.56 0.086
12.5" car 5.56 0.065
12.5" car 5.56 0.081
11.5" car 5.56 0.081
11.5" car 5.56 0.089
24" rifle 5.56 0.089
AR-15 Platform 7.62x39mm Conversion
20" rifle 7.62x39 0.12
20" rifle 7.62x39 0.125
16" car 7.62x39 0.078
16" car 7.62x39 0.081
16" car 7.62x39 0.0938
A note about port diameters and performance:
If you've already built an AR and are experiencing issues with the gas system (like over-gassing and excessive recoil or conversely, short strokes, failures to feed or cycle, or jams) it is strongly recommended you first attempt to replace the buffer with a lighter or heavier unit, followed by installing an adjustable gas block (discussed in the next section). Modifying the gas port diameter can have disastrous consequences, requiring a barrel swap. Because port diameters are often non-specific to barrel length and gas tube length as shown above, it is difficult to identify "the perfect diameter" for any one cartridge and setup.
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