I’m a little embarrassed to admit this, but I never gave an adjustable gas block on an AR-15 much thought until I was forced to use one recently while evaluating an ArmaLite M-15 VSR Versatile Sporting Rifle. I’m not talking about the three-position blocks like on a Ruger. I’m talking about the ones that you fully close and then slowly open until you get 100 percent reliability. Thoughtlessly I had dismissed them as one of those superfluous gizzies manufacturers added to have some feature—anything—to help separate their AR-15 from all the others on the market. I was wrong—really, really wrong.
Everything I had read indicated an adjustable block lets you “tune” the gun to your load or a suppressor. Hey, if I wasn’t using a suppressor and the gun was running, what needed tuning?
On the ArmaLite, the company recommends first closing the gas block adjustment fully and then opening it 2 1/2 turns. Next, you simply load one round in the magazine and fire it. If the bolt does not lock back when you shoot, turn the adjustment screw counterclockwise another 1/4 turn and repeat until the bolt consistently locks back.
ArmaLite recommends an additional 1/4 counterclockwise turn to ensure reliability. With Hornady’s .223 Rem. Superformance load, I was open 3 1/4 turns before tightening the setscrew. Every other load I shot cycled equally well at that setting, so that is where I left it. I was impressed with how much a difference tuning the gas block makes. Instead of simply tuning to the load, it reduces gun movement caused by recoil—a lot. It’s almost like having a muzzle brake without the added blast.
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