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The gas system of your AR-15 is an absolutely essential part of your rifle, and the AR-15 gas block is an essential part of the gas system. Just like almost every other part made for the AR-15, there are many different types of AR-15 gas blocks available. The right gas block may not only improve the aesthetics of your build, but it can also improve the function of your rifle. In fact, this small AR-15 part can actually reduce recoil and wear, increase reliability, give you the option to run suppressed or unsuppressed, and more. There are several important factors to keep in mind when shopping for the gas block you want to run on your custom AR15 build.
Gas blocks come in both standard and adjustable formats. Most standard gas blocks allow a set amount of gas to flow through the system. Some allow more gas in than necessary, called “over-gassing”. This is especially helpful when there is carbon buildup or when there is other debris that partially blocks the gas flow in your AR. With over-gassing, your gun can continue operating even with a partial blockage, which is nice with regards to reliability.
However, when higher temperatures combine with the excess gas from “over-gassing”, it can also create quite a bit of additional carbon residue that leads to a buildup. So essentially, over-gassing your AR “fixes” a problem that it also helps cause. Nevertheless, standard gas blocks have the advantage of being much more simple (no adjustments are needed) and easier on your wallet.
Adjustable gas blocks allow you to change the amount of gas flowing through the block itself. They are considered an upgrade to standard blocks and can actually take your gun to a whole new level of performance, depending on your needs. Proper use results in a cleaner system overall, less felt recoil, and less wear on your firearm.
On a basic level, there are a few adjustable gas block options, the “cut-off” type and the “bleed off” type.
AR-15 Gas blocks come in different sizes to fit onto the gas journal of various diameter barrels. For example, gas blocks with an inner diameter measuring 0.625” inches will typically be used for “Pencil” type barrels or thinner lightweight barrels.
“Standard” barrels, are generally considered to be medium-sized and are normally paired with gas blocks that have a 0.750” inch inner diameter. You’ll probably notice that 0.750” is a commonly available size.
Finally, some gas blocks are made to fit what are known as "Heavy" or “Bull" barrels or what are generally considered to be the thickest barrels for the given platform. Such a barrel is usually larger and heavier than a standard one. In most cases, 0.875” and 0.936” inner diameter gas blocks are the standard size used for these barrels.
CAUTION NOTE: When building a heavy/bull AR, you must be very selective of the handguard system that you will use. While most handguards easily slip over a standard low profile 0.750" gas-block, finding a handguard with a large enough internal diameter for 0.875" and larger gas-block can be difficult, and very expensive.