Adjustable gas blocks come in a number of forms. Some have 3 pre-set positions that the operator can select like “normal”, “adverse” for when your gun is really dirty and you don’t have time to clean it, and “off” for when you want to manually cycle the action. Some have settings for “suppressed” for when using a sound suppressor. These are most common on military rifles and one of the earliest was the Belgian FN-FAL. It’s gas block had a nut that could be unscrewed one click at a time which gradually exposed a gas port which would vent excess gas pressure. This threaded nut concept is useful because the operator could set it exactly where he wants it for his gun and his ammo instead of just having a “normal” or “adverse” setting. This ensures the gun is set to where it will run reliably without running excessively hard causing more recoil and wear on parts than necessary.
This concept has made it into the aftermarket parts industry as well for guns like the M1 Garand and AR-15 where adjustable gas plugs or gas blocks can be installed by the operator or a gunsmith. I’ll focus on the AR-15 adjustable gas blocks since they have become quite popular for those building custom guns or upgrading their current rifle. A few years ago, when adjustable gas blocks were relatively new on AR-15s, they simply consisted of a gas block with a screw in the side that would gradually cut off the gas flow from the barrel to the gas tube as you screwed it in. This is the simplest type to manufacture and quite a few companies make them now. Some of the most popular examples of these are made by JP Enterprises which have been around for years. PRI has also made one for a while and Wilson Combat and several others have recently come out with their own designs. These work well, but the screw isn’t secured in any way, so it could technically drift since it’s not locked in place. In practice, carbon builds up in the threads and normally keeps the screw setting stable. This style is still widely used and liked today, especially for competition shooters.
But many shooters want things that are locked down and can’t come loose during a competition or a self defense situation no matter what. So this led to adjustable gas blocks with lockable settings. Lockable settings can consist of a second screw that is tightened into the side of the adjustment screw to lock it into position once you got the setting where you want it. This is a simple approach that works and is used on some high quality gas blocks like those from Seekins Precision. The Seekins Precision Adjustable Gas Block uses a brass locking screw to secure the set screw. The most recent development are those with spring loaded detents which click into position every quarter turn as you adjust the set screw. Popular examples of these are made by Syrac Ordinance and SLR Rifleworks. The nice thing about this is that all you have to do is turn the set screw with an Allen wrench and it locks in place automatically as you go, so you don’t need to tighten a second screw and it stays very secure on the setting you want and it’s easier lock into position while under a handguard. Additionally, with audible click settings, you can keep tract of settings and adjust it by a known number of clicks for a certain type of ammo for example.
So what can you expect to get from using and adjustable gas block on your AR-15 or AR-10 type rifle? Many guns come over-gassed from the factory and run the bolt carrier group harder than it needs to, so it can be tuned down a bit to feel softer recoiling while still functioning properly. You also have the option of trying a different buffer system and optimizing the gas flow to work with that buffer. One of the most effective ways to use an adjustable gas block is to pair it with a lightweight bolt carrier and then tune down the gas to get the speed and reliability right. This makes for less reciprocating mass for less movement of the gun as it cycles and quicker sight picture recovery for the next shot. This concept was popularized by JP Enterprises as the Low Mass Operating System or LMOS and now several other manufacturers are making light weight bolt carriers as well. A fringe benefit of tuning down the gas is that it may run a bit cleaner with less gas going through the bolt carrier.
Not all ARs can benefit from and Adjustable Gas Block since some don’t have too much gas coming out of the gas port and you wouldn’t want to tune it down further. But for guns running very hard, or for those who want to use a light weight bolt carrier, it can be very useful. One thing to remember is that you can’t increase gas pressure with an adjustable gas block. The full pressure is determined by the gas port location and size in the barrel. An Adjustable Gas Block can only tune down gas pressure. But they do have a useful place in firearms design and are something you might want to consider if building or customizing an AR type rifle.